[clang] [Clang][AST] Fix MS Mangle concept uneval context template instantiation crash (PR #117845)

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https://github.com/zmodem approved this pull request.

The fix LGTM. I do wonder if the test should live in CodeGen/ instead, but I don't have a strong feeling about this (AST/ already seems to have some codegen tests).

https://github.com/llvm/llvm-project/pull/117845