[clang] [clang] constexpr built-in fma function. (PR #113020)

[Hubert Tong via cfe-commits]

hubert-reinterpretcast wrote:

A problem with the current patch is that it does not evaluate, even in constant expression contexts, cases that require rounding:
```cpp
extern constexpr float onepluszeroeps = __builtin_fmaf(__FLT_EPSILON__, .0f, 1.f);
extern constexpr float oneplushalfeps = __builtin_fmaf(__FLT_EPSILON__, .5f, 1.f);
```

```
<stdin>:2:24: error: constexpr variable 'oneplushalfeps' must be initialized by a constant expression
    2 | extern constexpr float oneplushalfeps = __builtin_fmaf(__FLT_EPSILON__, .5f, 1.f);
      |                        ^                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.
```

https://github.com/llvm/llvm-project/pull/113020