[clang] [clang] Mark `__builtin_convertvector` and `__builtin_shufflevector` as `constexpr`. (PR #112129)

[Nikolas Klauser via cfe-commits]

philnik777 wrote:

> You can directly view it from the Clang AST on Godbolt. For this patch, I don't believe additional tests are necessary.
> 
> 

Any bugfix should contain a regression test. Here you could simply check that `__has_constexpr_builtin` is true for these builtins, since that is the most obvious behaviour change.

https://github.com/llvm/llvm-project/pull/112129