[clang] [llvm] [clang] Add fixed point precision macros (PR #81207)

[via cfe-commits]

PiJoules wrote:

> I'm by no means a fixed-point expert, so this is a bit of a drive-by review.
> 
> One thing to note is that we shouldn't be using N1169 because TR 18037 is freely available: https://standards.iso.org/ittf/PubliclyAvailableStandards/c051126_ISO_IEC_TR_18037_2008.zip
> 
> The changes should come with a release note so that users know about them.

Replaced mentions of N1169 with TR 18037. Also updated clang release notes.

https://github.com/llvm/llvm-project/pull/81207