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

[Alex Voicu via cfe-commits]

AlexVlx wrote:

I'm not super thrilled about NOT having to introduce a type name, I'm not looking forward to the exciting world of `void foo(int __attribute__((ext_vector_type(4))) x)` replacing `void foo(int4 x)`.

I am extremely not at ease with flipping this to be a C++ attribute. Back in the dawn of time, C++ attributes were meant to be non-semantic / removable with no effect. This has kindof become muddled throughout the years, and we're probably moving away from that, and yes you have namespaced it, but I'd like such things to not be super cute and to remain clearly double prefix scary things.

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