Reinstate diagnostics for assignment from Int to unsigned types.

These had previously been removed in
9c2bc50 because of differences in the
diagnostics being emitted across different platforms.

It looks like we're always emitting the same diagnostic now, so restore
a specific message.

Resolves: rdar://problem/19677545

Author: rudkx

validation-test/stdlib/FixedPointDiagnostics.swift.gyb

@@ -2,13 +2,13 @@
 // RUN: %line-directive %t/main.swift -- %target-swift-frontend -typecheck -verify %t/main.swift
 
 func testUnaryMinusInUnsigned() {
-  var a: UInt8 = -(1) // expected-error {{}} expected-note * {{}} expected-warning * {{}}
+  var a: UInt8 = -(1) // expected-error {{cannot convert value of type 'Int' to specified type 'UInt8'}} expected-note * {{}} expected-warning * {{}}
 
-  var b: UInt16 = -(1) // expected-error {{}} expected-note * {{}} expected-warning * {{}}
+  var b: UInt16 = -(1) // expected-error {{cannot convert value of type 'Int' to specified type 'UInt16'}} expected-note * {{}} expected-warning * {{}}
 
-  var c: UInt32 = -(1) // expected-error {{}} expected-note * {{}} expected-warning * {{}}
+  var c: UInt32 = -(1) // expected-error {{cannot convert value of type 'Int' to specified type 'UInt32'}} expected-note * {{}} expected-warning * {{}}
 
-  var d: UInt64 = -(1) // expected-error {{}} expected-note * {{}} expected-warning * {{}}
+  var d: UInt64 = -(1) // expected-error {{cannot convert value of type 'Int' to specified type 'UInt64'}} expected-note * {{}} expected-warning * {{}}
 }
 
 // Int and UInt are not identical to any fixed-size integer type