[flang] Fix check for PRIVATE override of PUBLIC t.b.p.

[flang] Fix check for PRIVATE override of PUBLIC t.b.p.

A PRIVATE procedure binding in a derived type extension may not
be an override of a PUBLIC procedure binding.  Declaration checking
for this case was working only in the presence of an explicit
PUBLIC accessibility attribute, when it should be checking for the
absence of a PRIVATE accessibility attribute.

Differential Revision: https://reviews.llvm.org/D146577