[clang] [Clang][RISCV] Add operator overloading for builtin RVV vector types (PR #192722)

[Philip Reames via cfe-commits]

preames wrote:

Note: I have significantly reframed the patch description to reflect the fact that clang _already_ supports operations on RVV vectors when the riscv_rvv_vector_bits attribute is used.  I had managed not to realize this.  Oops.

I think this significantly simplified this review, and the follow on patches as all the major questions on e.g. comparison semantics are already made for the fixed vector cases, and we solely need to follow them for the scalable ones.  

https://github.com/llvm/llvm-project/pull/192722