[flang] Accept FINAL forward reference to separate module procedure

When one of a derived type's FINAL procedures is in a submodule,
its separate module procedure interface must necessarily be a
forward reference from the FINAL statement, as its interface
could not appear before the definition of the type.  The implementation
of FINAL procedure name resolution doesn't work for forward references;
replace it.

Differential Revision: https://reviews.llvm.org/D159035