[flang] Respect inaccessibility of type-bound ASSIGNMENT(=)

When a derived type has a PRIVATE type-bound generic binding for
a defined ASSIGNMENT(=), don't use it in scopes outside of the
module that defines the type.  We already get this case right
for other type-bound generics, including defined operators,
and for non-type-bound generic interfaces, but the check was
not applied for this case.

Differential Revision: https://reviews.llvm.org/D143826