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

[Shilei Tian via cfe-commits]

shiltian wrote:

I don't think to expose a string as the 2nd argument is a nice design, similar to my position to another PR that introduce atomic scope as a string.

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