[clang] [clang] Improve diagnostics for vector builtins (PR #125673)

[Fraser Cormack via cfe-commits]

frasercrmck wrote:

> There are a couple of things I thought I'd leave to the PR discussion, like:
> 
>     * Should we standardise the diagnostics a bit better? Specifically with regards to whether the possibility of "vector" needs to be made explicit when we say `a floating point type`, `an unsigned integer`? That's why I added `an integer or vector of integers` but now it stands out.
> 

The latest version of this PR does just that. I've tried to split up the error diagnostic so that it's more modular. Downstream I needed a way of accepting a scalar or vector of 'int' which felt like the straw that broke the camel's back.


https://github.com/llvm/llvm-project/pull/125673