Remove AST inclusion from Basic include

That's a cyclic dependency. NFC.

clang/include/clang/Basic/Thunk.h

@@ -15,10 +15,13 @@
 #ifndef LLVM_CLANG_BASIC_THUNK_H
 #define LLVM_CLANG_BASIC_THUNK_H
 
-#include "clang/AST/GlobalDecl.h"
+#include <cstdint>
+#include <cstring>
 
 namespace clang {
 
+class CXXMethodDecl;
+
 /// A return adjustment.
 struct ReturnAdjustment {
   /// The non-virtual adjustment from the derived object to its