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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> 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 only applies to *standard* attributes and has never been intended to apply to *vendor* attributes. Consider other type attributes like calling conventions on functions.

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