[flang] Special handling of ENTRY BIND(C,NAME=...)

We apply special symbol table scoping to top-level subroutine
and function names that have interoperable binding names, so
that it's possible to define the same subroutine/function name
more than once at the top level so long as their binding names
are distinct.  But we don't use those scoping techniques for
ENTRY statement symbols with interoperable binding names,
which can lead to bogus semantic errors when the same ENTRY
name is defined multiple times with distinct binding names.

Differential Revision: https://reviews.llvm.org/D134401