[SwiftCompilerSources] Disfavor overload of == that takes StringRef

This overload is disfavored to make sure that it's only used for cases
that don't involve literals, for that `==(StringRef, StaticString) -> Bool`
is preferred. Otherwise these overloads are going to be ambiguous
because both `StringRef`, `StaticString` conform to `ExpressibleByStringLiteral`.

Consider the following example:

```swift
func test(lhs: StringRef) {
  lhs == "<<test>>"
}
```

The type-checker used to pick `==(StringRef, StringRef)` overload in this
case because it has homogenous parameter types but this is no longer the
case because this behavior was too aggressive and led to sub-optimal choices
by completely skipping other viable overloads.

Since `StaticString` already represents literals it's better to use
a standard library type and reserve the `(StringRef, StringRef)`
overload to when the literals are not involved.

Resolves: rdar://154719565

Author: xedin

SwiftCompilerSources/Sources/Basic/Utils.swift

@@ -110,6 +110,10 @@ public struct StringRef : CustomStringConvertible, NoReflectionChildren, Express
     }
   }
 
+  /// This overload is disfavored to make sure that it's only used for cases that don't involve literals, for that
+  /// `==(StringRef, StaticString) -> Bool` is preferred. Otherwise these overloads are
+  /// going to be ambiguous because both StringRef, StaticString conform to `ExpressibleByStringLiteral`.
+  @_disfavoredOverload
   public static func ==(lhs: StringRef, rhs: StringRef) -> Bool {
     let lhsBuffer = UnsafeBufferPointer<UInt8>(start: lhs._bridged.data, count: lhs.count)
     let rhsBuffer = UnsafeBufferPointer<UInt8>(start: rhs._bridged.data, count: rhs.count)