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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> > @zahiraam @AaronBallman a different option would be to add a signed vs unsigned storage option to the `OPTION` and `BENIGN_ENUM_LANGOPT` macros so we can store negative enumerations safely
> 
> I think I would prefer this solution. We need to be able to set the evaluation method to a value (-1) when it can't be known from the target or when the value of `ffp-eval-method` is inconsistent with the target.

Could we shift all the values, so `FEM_Indeterminable` is `0`?

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