HexFloat floating-point support

[AaronBallman]

RFC: https://discourse.llvm.org/t/rfc-hexfloat-floating-point-support/75833

[AaronBallman]

Original RFC posted in 2023 but has never been formally accepted and there are recent questions from maintainers about whether we want to accept the RFC or not.

[AaronBallman]

This RFC was discussed at the Clang Area Team meeting on Jul 16, the minutes can be found at: https://docs.google.com/document/d/13-68BCOt8kH5k_56fURBi7u7Ap97Z0EshgkJXDudbJM/edit?pli=1&tab=t.0

Based on the last comment on the thread at the time, we were waiting on the author to respond with updates. Corentin posted a request for a status update, so this is still waiting on the author: https://discourse.llvm.org/t/rfc-hexfloat-floating-point-support/75833/30