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

[Oliver Hunt via cfe-commits]

ojhunt wrote:

@zahiraam we started diagnosing size/type mismatch of bitfields vs preferred_type in, which exposed that FPEvalMethodKind based options can't store FEM_Indeterminable https://github.com/llvm/llvm-project/pull/116785

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