Add Equatable and Hashable conformance to GUID. (#84792)

On Windows, `GUID` is a currency type and (along with its various
typedefs) is used pervasively Windows offers `IsEqualGUID()` and
`UuidHash()` for comparing and hashing them, respectively, but
`IsEqualGUID()` is a macro in C mode and `UuidHash()` only provides a
16-bit hash. We should provide conformance to these protocols in the
WinSDK overlay to provide equivalent functionality in Swift.

stdlib/public/Windows/WinSDK.swift

@@ -319,3 +319,31 @@ func _convertWindowsBoolToBool(_ b: WindowsBool) -> Bool {
   return b.boolValue
 }
 
+// GUID
+
+extension GUID {
+  @usableFromInline @_transparent
+  internal var uint128Value: UInt128 {
+    unsafe withUnsafeBytes(of: self) { buffer in
+      // GUID is 32-bit-aligned only, so use loadUnaligned().
+      unsafe buffer.baseAddress!.loadUnaligned(as: UInt128.self)
+    }
+  }
+}
+
+// These conformances are marked @retroactive because the GUID type nominally
+// comes from the _GUIDDef clang module rather than the WinSDK clang module.
+
+extension GUID: @retroactive Equatable {
+  @_transparent
+  public static func ==(lhs: Self, rhs: Self) -> Bool {
+    lhs.uint128Value == rhs.uint128Value
+  }
+}
+
+extension GUID: @retroactive Hashable {
+  @_transparent
+  public func hash(into hasher: inout Hasher) {
+    hasher.combine(uint128Value)
+  }
+}

test/stdlib/WinSDK_GUID.swift

@@ -1,4 +1,7 @@
-// RUN: %target-build-swift %s
+// RUN: %target-build-swift %s -o %t.exe
+// RUN: %target-codesign %t.exe
+// RUN: %target-run %t.exe
+// REQUIRES: executable_test
 // REQUIRES: OS=windows-msvc
 
 // Make sure that importing WinSDK brings in the GUID type, which is declared in
@@ -7,3 +10,13 @@
 import WinSDK
 
 public func usesGUID(_ x: GUID) {}
+
+// Make sure equating and hashing GUIDs works.
+
+let guid: GUID = GUID_NULL
+assert(guid == guid)
+assert(guid.hashValue == guid.hashValue)
+
+let guid2: GUID = IID_IUnknown
+assert(guid != guid2)
+assert(guid.hashValue != guid2.hashValue) // well, probably