[Sema] Comparison of pointers to complete and incomplete types

Clang is missing one of the conditions for C99 6.5.9p2, where comparison
between pointers must either both point to incomplete types or both
point to complete types. This patch adds an extra check to the clause
where two pointers are of compatible types.

This only applies to C89/C99; the relevant part of the standard was
rewritten for C11.

Differential Revision: https://reviews.llvm.org/D79945