Update the error message when subscripting String with Int. (#27069)

Instead of referring to a doc comment that isn't really accessible to the
developer, let's be accurate about what type they should be using here.

Author: natecook1000

stdlib/public/core/UnavailableStringAPIs.swift.gyb

@@ -55,15 +55,15 @@ extension String {
 ${stringSubscriptComment}
   @available(
     *, unavailable,
-    message: "cannot subscript String with an Int, see the documentation comment for discussion")
+    message: "cannot subscript String with an Int, use a String.Index instead.")
   public subscript(i: Int) -> Character {
     Builtin.unreachable()
   }
 
 ${stringSubscriptComment}
   @available(
     *, unavailable,
-    message: "cannot subscript String with an integer range, see the documentation comment for discussion")
+    message: "cannot subscript String with an integer range, use a String.Index range instead.")
   public subscript<R: RangeExpression>(bounds: R) -> String where R.Bound == Int {
     Builtin.unreachable()
   }

test/Constraints/diagnostics.swift

@@ -683,7 +683,7 @@ func r23641896() {
   var g = "Hello World"
   g.replaceSubrange(0...2, with: "ce")  // expected-error {{cannot convert value of type 'ClosedRange<Int>' to expected argument type 'Range<String.Index>'}}
 
-  _ = g[12]  // expected-error {{'subscript(_:)' is unavailable: cannot subscript String with an Int, see the documentation comment for discussion}}
+  _ = g[12]  // expected-error {{'subscript(_:)' is unavailable: cannot subscript String with an Int, use a String.Index instead.}}
 
 }
 

test/stdlib/StringDiagnostics.swift

@@ -7,19 +7,19 @@ import Foundation
 // Common pitfall: trying to subscript a string with integers.
 func testIntSubscripting(s: String, i: Int) {
   // FIXME swift-3-indexing-model: test new overloads of ..<, ...
-  _ = s[i] // expected-error{{'subscript(_:)' is unavailable: cannot subscript String with an Int, see the documentation comment for discussion}}
-  _ = s[17] // expected-error{{'subscript(_:)' is unavailable: cannot subscript String with an Int, see the documentation comment for discussion}}
-  _ = s[i...i] // expected-error{{'subscript(_:)' is unavailable: cannot subscript String with an integer range, see the documentation comment for discussion}}
-  _ = s[17..<20] // expected-error{{'subscript(_:)' is unavailable: cannot subscript String with an integer range, see the documentation comment for discussion}}
-  _ = s[17...20] // expected-error{{'subscript(_:)' is unavailable: cannot subscript String with an integer range, see the documentation comment for discussion}}
+  _ = s[i] // expected-error{{'subscript(_:)' is unavailable: cannot subscript String with an Int, use a String.Index instead.}}
+  _ = s[17] // expected-error{{'subscript(_:)' is unavailable: cannot subscript String with an Int, use a String.Index instead.}}
+  _ = s[i...i] // expected-error{{'subscript(_:)' is unavailable: cannot subscript String with an integer range, use a String.Index range instead.}}
+  _ = s[17..<20] // expected-error{{'subscript(_:)' is unavailable: cannot subscript String with an integer range, use a String.Index range instead.}}
+  _ = s[17...20] // expected-error{{'subscript(_:)' is unavailable: cannot subscript String with an integer range, use a String.Index range instead.}}
 
-  _ = s[Range(i...i)] // expected-error{{'subscript(_:)' is unavailable: cannot subscript String with an integer range, see the documentation comment for discussion}}
-  _ = s[Range(17..<20)] // expected-error{{'subscript(_:)' is unavailable: cannot subscript String with an integer range, see the documentation comment for discussion}}
-  _ = s[Range(17...20)] // expected-error{{'subscript(_:)' is unavailable: cannot subscript String with an integer range, see the documentation comment for discussion}}
+  _ = s[Range(i...i)] // expected-error{{'subscript(_:)' is unavailable: cannot subscript String with an integer range, use a String.Index range instead.}}
+  _ = s[Range(17..<20)] // expected-error{{'subscript(_:)' is unavailable: cannot subscript String with an integer range, use a String.Index range instead.}}
+  _ = s[Range(17...20)] // expected-error{{'subscript(_:)' is unavailable: cannot subscript String with an integer range, use a String.Index range instead.}}
 
-  _ = s[Range(i...i)] // expected-error{{'subscript(_:)' is unavailable: cannot subscript String with an integer range, see the documentation comment for discussion}}
-  _ = s[Range(17..<20)] // expected-error{{'subscript(_:)' is unavailable: cannot subscript String with an integer range, see the documentation comment for discussion}}
-  _ = s[Range(17...20)] // expected-error{{'subscript(_:)' is unavailable: cannot subscript String with an integer range, see the documentation comment for discussion}}
+  _ = s[Range(i...i)] // expected-error{{'subscript(_:)' is unavailable: cannot subscript String with an integer range, use a String.Index range instead.}}
+  _ = s[Range(17..<20)] // expected-error{{'subscript(_:)' is unavailable: cannot subscript String with an integer range, use a String.Index range instead.}}
+  _ = s[Range(17...20)] // expected-error{{'subscript(_:)' is unavailable: cannot subscript String with an integer range, use a String.Index range instead.}}
 }
 
 func testNonAmbiguousStringComparisons() {

test/stdlib/UnavailableStringAPIs.swift.gyb

@@ -8,9 +8,9 @@ func test_StringSubscriptByInt(
   r1: Range<Int>,
   r2: ClosedRange<Int>
 ) {
-  _ = x[i] // expected-error {{'subscript(_:)' is unavailable: cannot subscript String with an Int, see the documentation comment for discussion}} {{none}}
-  _ = x[r1] // expected-error {{'subscript(_:)' is unavailable: cannot subscript String with an integer range, see the documentation comment for discussion}} {{none}}
-  _ = x[r2] // expected-error {{'subscript(_:)' is unavailable: cannot subscript String with an integer range, see the documentation comment for discussion}} {{none}}
+  _ = x[i] // expected-error {{'subscript(_:)' is unavailable: cannot subscript String with an Int, use a String.Index instead.}} {{none}}
+  _ = x[r1] // expected-error {{'subscript(_:)' is unavailable: cannot subscript String with an integer range, use a String.Index range instead.}} {{none}}
+  _ = x[r2] // expected-error {{'subscript(_:)' is unavailable: cannot subscript String with an integer range, use a String.Index range instead.}} {{none}}
 }
 
 func test_UTF8View(s: String.UTF8View, i: String.UTF8View.Index, d: Int) {