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

[Oliver Hunt via cfe-commits]

ojhunt wrote:

@AaronBallman @zahiraam  given we do not support FEM_Indeterminate, and there appears to be no way to actually get clang into the state anyway, I think that the correct thing to do is to remove it and work out had to get this to cooperate with the OPTION macro if we ever do add support for it, does this seem reasonable?

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