[clang] [Clang] prevent crash on invalid nested name specifiers with a single colon (PR #169246)

[Oleksandr T. via cfe-commits]

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@@ -1063,7 +1063,8 @@ Parser::isCXXDeclarationSpecifier(ImplicitTypenameContext AllowImplicitTypename,
       return TPResult::False;
     }
 
-    if (Next.isNot(tok::coloncolon) && Next.isNot(tok::less)) {
+    if (Next.isNot(tok::coloncolon) && Next.isNot(tok::less) &&
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a-tarasyuk wrote:

Thanks for your feedback, @AaronBallman. I’ve added the helper.

https://github.com/llvm/llvm-project/pull/169246