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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

Is there an associated RFC (https://llvm.org/docs/RFCProcess.html) on Discourse discussing this PR? If not, I think it would make sense to post one so the community can discuss the design. (For example, should this work in constant expression contexts or should this be disallowed? Is it exposed for both C and C++? etc) One thing that would help is a discussion of why this builtin should be exposed -- is this needed for implementing something in the STL, etc?

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