[flang] Document and warn about an extension

Standard Fortran allows type-bound procedure bindings to only
be called, and disallows them from being used in other contexts
where a procedure name can be: as the target of a procedure pointer
assignment statement, and as an actual argument that corresponds
to a dummy procedure.  So long as the interfaces match, there's
no good reason for these uses to be errors, and there some obvious
use cases in polymorphic programming.  So emit portability warnings
rather than errors, and document this usage as an extension.

Differential Revision: https://reviews.llvm.org/D139127