Adjust compiler version checks around WinSDK.GUID.Wrapper. (#1483)

This PR ensures we don't start spuriously warning if Swift Testing is
built as a package with the (eventual/hypothetical) Swift 6.3.1
toolchain, and uses the built-in `Equatable` and `Hashable` conformances
on `GUID` when built with 6.3.0 (because those conformances have landed
everywhere now.)

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: grynspan

Sources/Overlays/_Testing_WinSDK/Support/Additions/GUIDAdditions.swift

@@ -17,7 +17,7 @@ extension GUID {
   ///
   /// - Bug: This type will become obsolete once we can use the `Equatable` and
   ///   `Hashable` conformances added to the WinSDK module in Swift 6.3.
-#if compiler(>=6.3.1) && DEBUG
+#if compiler(>=6.4) && DEBUG
   @available(*, deprecated, message: "GUID.Wrapper is no longer needed and can be removed.")
 #endif
   struct Wrapper: Sendable, RawRepresentable {
@@ -32,7 +32,7 @@ extension GUID.Wrapper: Equatable, Hashable, CustomStringConvertible {
     self.init(rawValue: rawValue)
   }
 
-#if compiler(<6.3.1)
+#if compiler(<6.3)
   private var _uint128Value: UInt128 {
     withUnsafeBytes(of: rawValue) { buffer in
       buffer.baseAddress!.loadUnaligned(as: UInt128.self)
@@ -46,10 +46,10 @@ extension GUID.Wrapper: Equatable, Hashable, CustomStringConvertible {
   func hash(into hasher: inout Hasher) {
     hasher.combine(_uint128Value)
   }
+#endif
 
   var description: String {
     String(describing: rawValue)
   }
-#endif
 }
 #endif