[Clang] prevent crash on invalid nested name specifiers with a single colon (#169246) Fixes #167905 --- This patch addresses an issue where invalid nested name specifier sequences containing a single colon (`a:c::`) could be treated during recovery as valid scope specifiers, which in turn led to a crash https://github.com/llvm/llvm-project/blob/c543615744d61e0967b956c402e310946d741570/clang/lib/Parse/ParseExprCXX.cpp#L404-L418 For malformed inputs like `a:c::`, the single colon recovery incorrectly triggers and produces an `annot_cxxscope`. When tentative parsing later runs https://github.com/llvm/llvm-project/blob/996213c6ea0dc2e47624c6b06c0833a882c1c1f7/clang/lib/Parse/ParseTentative.cpp#L1739-L1740 the classifier returns `Ambiguous`, which doesn't stop parsing. The parser then enters the https://github.com/llvm/llvm-project/blob/996213c6ea0dc2e47624c6b06c0833a882c1c1f7/clang/lib/Parse/ParseTentative.cpp#L1750-L1752 and consumes the invalid scope annotation, eventually reaching `EOF` and crashing.
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