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

[Zahira Ammarguellat via cfe-commits]

zahiraam wrote:

@ojhunt  In https://github.com/llvm/llvm-project/issues/137600 there are a list of a few temporary PRs that were merged in to silence the warning. These need to be reverted after this one gets merged in. Do you want to revert them or you want me to do that?

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