[clang][NFC] Add a notice to desugarForDiagnostic

`desugarForDiagnostic` only sets ShouldAKA to true if desugaring
happens, otherwise ShouldAKA is left intact and might be uninitialized.

Victims (including me):

llvm/llvm-project@25bf8cb

llvm/llvm-project@0e8384a

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D152880

Author: zyn0217

clang/include/clang/AST/ASTDiagnostic.h

@@ -34,7 +34,8 @@ namespace clang {
       ArrayRef<intptr_t> QualTypeVals);
 
   /// Returns a desugared version of the QualType, and marks ShouldAKA as true
-  /// whenever we remove significant sugar from the type.
+  /// whenever we remove significant sugar from the type. Make sure ShouldAKA
+  /// is initialized before passing it in.
   QualType desugarForDiagnostic(ASTContext &Context, QualType QT,
                                 bool &ShouldAKA);
 }  // end namespace clang

clang/lib/AST/ASTDiagnostic.cpp

@@ -25,7 +25,8 @@
 using namespace clang;
 
 // Returns a desugared version of the QualType, and marks ShouldAKA as true
-// whenever we remove significant sugar from the type.
+// whenever we remove significant sugar from the type. Make sure ShouldAKA
+// is initialized before passing it in.
 QualType clang::desugarForDiagnostic(ASTContext &Context, QualType QT,
                                      bool &ShouldAKA) {
   QualifierCollector QC;