[clang][NFC] Add a notice to desugarForDiagnostic

[zyounan]

Summary

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

Victims (including me):

https://github.com/llvm/llvm-project/commit/25bf8cb3c0e3c41231289a6ff0a37b6d49b24011

https://github.com/llvm/llvm-project/commit/0e8384a0fe4f03d60cd92aba1cae074512481ca2

[aaron.ballman]

LGTM, thank you for the extra documentation, that is a surprising behavior

clang/include/clang/AST/ASTDiagnostic.h

void FormatASTNodeDiagnosticArgument(
       StringRef Modifier,
       StringRef Argument,
       ArrayRef<DiagnosticsEngine::ArgumentValue> PrevArgs,
       SmallVectorImpl<char> &Output,
       void *Cookie,
       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
 
 #endif

clang/lib/AST/ASTDiagnostic.cpp

 #include "clang/AST/TemplateBase.h"
 #include "clang/AST/Type.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/Support/raw_ostream.h"
 
 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;
 
   while (true) {
     const Type *Ty = QC.strip(QT);
 
     // Don't aka just because we saw an elaborated type...