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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> It completely changes the type, it's much more aggressive than a type attribute? sizeof is no longer the same

We already have type-level vector attributes, like the vector size or NEON attributes. Some of them also have `clang::` spellings.

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