[clang] [clang] Remove FEM_Indeterminable (PR #137661)

[Oliver Hunt via cfe-commits]

ojhunt wrote:

> > I think this is a reasonable way forward for right now; we can bring back the enumerator once we're actually using it. WDYT @zahiraam ?
> 
> I tend to think so too. But I am waiting for a response from the math library people internally to make sure that we don't loose compatibility with `icc` doing this.

@zahiraam @AaronBallman r+'d this, and it's causing a bunch of warning noise for folk, so I'd like to land this soon - do you have any idea when icc folk can get back to you? Also, if they can find an issue can you find out how they're getting clang into the FEM_Indeterminate state? I can't find any way to trigger it, and as far as I can make out even if we could I believe we would currently set `__FLT_EVAL_METHOD__` to 7 so I'm really not sure how that could break anything

https://github.com/llvm/llvm-project/pull/137661