https://github.com/llvm/llvm-project/issues/149993

This involved a little bit of yak shaving because one of the new tests
depends on MPC, and we didn't have targets for it yet, so I ended up
needing to add a similar setup to what we have for MPFR.

`cpp::is_complex_type_same<T1, T2>` is a function, so we need
parentheses in order to call it. Otherwise the expression is treated
like a function pointer which is always true in this boolean context.

This PR aims to add the groundwork to test the precision of libc complex
functions against MPC. I took `cargf` as a test to verify that the infra
works fine.