[clang] [clang] Remove FEM_Indeterminable (PR #137247)

[Zahira Ammarguellat via cfe-commits]

zahiraam wrote:

> > I've reverted, not sure why I thought it had been reviewed?
> 
> Thanks! And I'm happy enough blaming GitHub's poor Ux for all confusion. :-)
> 
> @zahiraam -- any chance you can look into this?

Yes I can in a couple of days. @ojhunt I hope this is OK?

https://github.com/llvm/llvm-project/pull/137247