Address GUID type definition errors (#85196)

In https://github.com/swiftlang/swift/pull/84792 we have added
implementation to `Equatable` and `Hashable` there are two issues with
this.
1. Uses of type `GUID` would be emitted in the `swiftinterface` file as
`_GUIDDef._GUID` since it is an external type. but the type name in the
imported header file is `GUID` and not `_GUID`
2. When compiling using `-cxx-interoperability-mode=default`, there are
duplicate definition errors for `==` since the c++ header file defines
the same operator.

Proposed changes:
1. Add a type alias `typealias _GUID = GUID` to address the naming
mismatch in the generated interface file
2. Add conditional definitions of the equatable implementation to avoid
duplicate definitions.

---------

Co-authored-by: Jonathan Grynspan <grynspan@me.com>

stdlib/public/Windows/WinSDK.swift

@@ -335,8 +335,12 @@ extension GUID {
 // comes from the _GUIDDef clang module rather than the WinSDK clang module.
 
 extension GUID: @retroactive Equatable {
+  // When C++ interop is enabled, Swift imports a == operator from guiddef.h
+  // that conflicts with the definition of == here, so we've renamed it to
+  // __equals to avoid the conflict.
   @_transparent
-  public static func ==(lhs: Self, rhs: Self) -> Bool {
+  @_implements(Equatable, ==(_:_:))
+  public static func __equals(lhs: Self, rhs: Self) -> Bool {
     lhs.uint128Value == rhs.uint128Value
   }
 }