[clang] [llvm] Add f8E4M3 IEEE 754 type to llvm (PR #97179)

[Alexander Pivovarov via cfe-commits]

apivovarov wrote:

Thorsten, Matt, What you think about adding the following tests?
```c
// E4M3 <-> E5M2
ConvertE4M3ToE5M2
ConvertE5M2ToE4M3

// E4M3 <-> E4M3FN
ConvertE4M3ToE4M3FN
ConvertE4M3FNToE4M3
```
@tschuett @arsenm

https://github.com/llvm/llvm-project/pull/97179