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

[Oliver Hunt via cfe-commits]

ojhunt wrote:

@zahiraam none of these actually try to set FEM_Indeterminate, so far I cannot find any way to make clang ever set that state and I can’t find a way for source code to do so either - it looks like in the past the fpcontrol pragma could induce it, but none of the examples still do so in clang _or_ gcc.

I really think the correct thing to do is to remove FEM_Indeterminate because even if you could currently set the clang fp eval mode it would not be presented as -1 in __FLT_EVAL_METHOD__ because of the problem this warning is complaining about (ie clang has not been able to produce __FLT_EVAL_METHOD__==-1 in a long time)

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