[clang] [RFC][clang] Add `__builtin_convert_from_arbitrary_fp` (PR #212647)

[Matt Arsenault via cfe-commits]

arsenm wrote:

I think there are 2 issues here:
1. The ergonomics of the string argument are poor. This either should be an enum for the format, or use an explicit type suffix for the combination. The explicit type suffix is somewhat more practical, because then you can write a __has_builtin check to see if a particular format is required. The enum would require some kind of other support check side channel
2. Should probably make it an elementwise builtin for vectors

https://github.com/llvm/llvm-project/pull/212647