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

[nicole mazzuca via llvm-commits]

strega-nil 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
```

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