[flang] Extend characterization & checking for procedure bindings

Procedure bindings with explicit interfaces don't work when the
interface is shadowed by a generic interface of the same name,
and can produce spurious semantic error messages.  Extend the
characterization and checking code for such things, and the utility
functionns on which they depend, to dig through generics when they
occlude interface-defining subprograms.  This is done on demand in
checking code, not once during name resolution, because the
procedures in question may also be forward-referenced.

Differential Revision: https://reviews.llvm.org/D131105

diff --git a/flang/include/flang/Semantics/symbol.h b/flang/include/flang/Semantics/symbol.h
index e79f8ab6503e..0b03bf06eb73 100644
--- a/flang/include/flang/Semantics/symbol.h