[flang] Allow mixed association of procedure pointers and targets

Section 10.2.2.4, paragraph 3 states that a procedure pointer with an explicit
interface must have the same characteristics as its target.  Previously, we
interpreted this as disallowing such pointers to point to procedures with
implicit interfaces.  But several other compilers allow this.

We make an exception for the case where the explicit interface cannot be
called via an implicit interface.

This change makes us allow this, also

Differential Revision: https://reviews.llvm.org/D119404