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

[Oliver Hunt via cfe-commits]

ojhunt 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

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