[libc] [llvm] [libc][complex] Testing infra for MPC (PR #121261)

[Shourya Goel via llvm-commits]

Sh0g0-1758 wrote:

@nickdesaulniers @lntue I have added a shared library with functions and classes that are needed both by `MPCWrapper` and `MPFRWrapper`. I earlier thought of simply exporting the definitions into `MPFRWrapper.h` but that would necessitate having `mpfr_inc.h` in the header file, which ofcourse would be highly undesirable. It was also leading to some nasty bugs during the full build and so decided to go with the shared library approach. 

https://github.com/llvm/llvm-project/pull/121261