[clang] [Clang] Fix assertion when instantiating a matrix type with an invalid element type (PR #224331)

[Ambrose Leeb via cfe-commits]

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

LGTM, but since this also now checks for `_BitInt` after instantiation, can you add a test for invalid `BitInt` element types as well?

https://github.com/llvm/llvm-project/pull/224331