AST: Avoid reading an unitialized value

Desugar doesn't necessarily initialize ShouldAKA, but as of r241542 it
may read it. Fix the misuse of the API and initialize this before
passing it in.

Found by ubsan.

llvm-svn: 241705

clang/lib/AST/ASTDiagnostic.cpp

@@ -192,8 +192,8 @@ ConvertTypeToDiagnosticString(ASTContext &Context, QualType Ty,
     if (CompareCanTy == CanTy)
       continue;  // Same canonical types
     std::string CompareS = CompareTy.getAsString(Context.getPrintingPolicy());
-    bool aka;
-    QualType CompareDesugar = Desugar(Context, CompareTy, aka);
+    bool ShouldAKA = false;
+    QualType CompareDesugar = Desugar(Context, CompareTy, ShouldAKA);
     std::string CompareDesugarStr =
         CompareDesugar.getAsString(Context.getPrintingPolicy());
     if (CompareS != S && CompareDesugarStr != S)