[clang] [Clang] Fix crash when type-name is combined with class specifier in template argument (PR #191689)

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TPPPP72 wrote:

> @mizvekov yeah, the issue is that when parsing `A class B::C` the parsing of `B::C` is performed in the declscope on the assumption that it is _the_ type specifier, which isn't correct. I'm going to push a PR momentarily that addresses this in what I think is a reasonable way (sorry @TPPPP72 it's not a trivial path from what I think is the actual failure point to the symptom in the bug, I really should have thought more about why clearing the scope was making the assertion go away).

Okay, once you've finished fixing it, just close this pull request and related issue.

https://github.com/llvm/llvm-project/pull/191689