[flang] BIND(C,NAME=...) corrections

The Fortran standard's various restrictions on the use of BIND(C)
often depend more on the presence or absence of an explicit NAME=
specification rather than on its value, but semantics and module
file generation aren't making distinctions between explicit NAME=
specifications that happen to match the default name and declarations
that don't have NAME=.  Tweak semantics and module file generation
to conform, and also complain when named BIND(C) attributes are
erroneously applied to entities that can't support them, like
ABSTRACT interfaces.

Differential Revision: https://reviews.llvm.org/D145107