[compiler-rt] [builtins] Start to refactor int to fp conversion functions to use a common implementation (PR #66903)

[Alex Bradbury via llvm-commits]

asb wrote:

> This seems to have broken the build under MSVC:
> 
> ```
> C:\src\llvm-project\compiler-rt\lib\builtins\int_to_fp_impl.inc(36): error C2051: case expression not constant
> C:\src\llvm-project\compiler-rt\lib\builtins\int_to_fp_impl.inc(39): error C2051: case expression not constant
> ```

Thanks for fixing, I'm sorry I'd missed this - seems I had an issue with my email filter rules.

I'm really surprised the code in int_to_fp.h was problematic but fp_trunc_impl.inc isn't.

https://github.com/llvm/llvm-project/pull/66903