[clang] [clang] Macro for constant rounding mode (PR #92699)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

Oh, I somehow thought the macro was part of the spec; reading again, I guess it isn't, it's just an attempt to implement the spec.

We probably want some feedback from libc implementers to check if this is what they want.  I don't really want to end up in a situation where we end up implementing this, but then nobody ends up using it.  Or we end up with two almost identical macros for the same thing.

https://github.com/llvm/llvm-project/pull/92699