[clang] [MIPS][clang] make `_Complex {integer}` returns match GCC (PR #212119)

[Folkert de Vries via cfe-commits]

folkertdev wrote:

Yes, but for a different problem that occurs when the argument slots run out and can only fit half of a complex float or double. Clang and GCC diverged there too which I have now fixed.

I've now validated the implementation with https://github.com/folkertdev/powerpc-complex-abi-validation which compiles various test programs with GCC and Clang and checks that values make it from one side to the other. 

https://github.com/llvm/llvm-project/pull/212119