[clang] [Clang] Treat `ext_vector_type` as a regular type attribute (PR #130177)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

I suppose I could try to introduce the RegularKeyword usage that the other patch used? That would make it like `__ext_vector_type` I think. (And honestly, while I'm at it, do you think we could drop the `ext`?)

https://github.com/llvm/llvm-project/pull/130177