[clang] [clang] Implement CWG2851: floating-point conversions in converted constant expressions (PR #90387)

[Vlad Serebrennikov via cfe-commits]

https://github.com/Endilll commented:

I'm fine with the way DR test is written, but I'm not qualified to review the contents on the test, unfortunately.

I'd like to point to related Core issues [CWG2836](https://cplusplus.github.io/CWG/issues/2836.html) and [CWG2864](https://cplusplus.github.io/CWG/issues/2864.html).

You also need to run `clang/www/make_cxx_dr_status` to update the DR status page.

https://github.com/llvm/llvm-project/pull/90387