Rename the new _GUID clang module to _GUIDDef.

This PR changes the name of the clang module introduced in #83914 from `_GUID`
to `_GUIDDef`. This change is necessary because the Windows SDK declares a type
named `_GUID` and Swift gets confused when a module and a type have the same
name.

This confusion is the motivation for [SE-0491](https://github.com/swiftlang/swift-evolution/blob/main/proposals/0491-module-selectors.md),
but we can't just wait for a fix because the new module is a source-breaking
change and clients won't be able to adopt the proposed syntax for some time.

Author: grynspan

stdlib/public/Platform/winsdk_shared.modulemap

@@ -10,7 +10,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-module _GUID {
+module _GUIDDef {
   header "guiddef.h"
   export *
 }

stdlib/public/Windows/WinSDK.swift

@@ -11,7 +11,7 @@
 //===----------------------------------------------------------------------===//
 
 @_exported import ucrt
-@_exported import _GUID
+@_exported import _GUIDDef
 @_exported import WinSDK // Clang module
 
 // WinBase.h