Don't add constraints for invalid subscript declarations.

This fixes the crash in sr-114. Adding constraints for the invalid operator decl means constraining to the error type, which fails an
assertion later on while binding an overload choice. In all the normal function/method overload choice cases, if the decl is invalid
that choice gets skipped (never generated), so this is just another case of the existing way of doing things

Author: gregomni

lib/Sema/CSGen.cpp

@@ -1517,8 +1517,11 @@ namespace {
 
     Type visitSubscriptExpr(SubscriptExpr *expr) {
       ValueDecl *decl = nullptr;
-      if (expr->hasDecl())
+      if (expr->hasDecl()) {
         decl = expr->getDecl().getDecl();
+        if (decl->isInvalid())
+          return Type();
+      }
       return addSubscriptConstraints(expr, expr->getBase(), expr->getIndex(),
                                      decl);
     }

test/decl/subscript/subscripting.swift

@@ -259,3 +259,18 @@ func testSubscript1(s2 : SubscriptTest2) {
   let b = s2[1, "foo"] // expected-error {{cannot subscript a value of type 'SubscriptTest2' with an index of type '(Int, String)'}}
   // expected-note @-1 {{expected an argument list of type '(String, String)'}}
 }
+
+// sr-114 & rdar://22007370
+
+class Foo {
+    subscript(key: String) -> String { // expected-note {{'subscript' previously declared here}}
+        get { a } // expected-error {{use of unresolved identifier 'a'}}
+        set { b } // expected-error {{use of unresolved identifier 'b'}}
+    }
+    
+    subscript(key: String) -> String { // expected-error {{invalid redeclaration of 'subscript'}}
+        get { a } // expected-error {{use of unresolved identifier 'a'}}
+        set { b } // expected-error {{use of unresolved identifier 'b'}}
+    }
+}
+