[flang] Accept POINTER followed by INTERFACE

As is already supported for dummy procedures, we need to also accept
declarations of procedure pointers that consist of a POINTER attribute
statement followed by an INTERFACE block.  (The case of an INTERFACE
block followed by a POINTER statement already works.)

While cleaning this case up, adjust the utility predicate IsProcedurePointer()
to recognize it (namely a SubprogramDetails symbol with Attr::POINTER)
and delete IsProcName().  Extend tests, and add better comments to
symbol.h to document the two ways in which procedure pointers are
represented.

Differential Revision: https://reviews.llvm.org/D125139