[flang] Better C_LOC and C_ASSOCIATED in flang/module

The place-holding implementation of C_LOC just didn't work
when used with our more complete semantic checking, specifically
in the case of a polymorphic argument; convert it to an external
function with an implicit interface.  C_ASSOCIATED needs to be
a generic interface with specific implementations for C_PTR and
C_FUNPTR.

Differential Revision: https://reviews.llvm.org/D94714