[flang] Warn about local names that are the same as their enclosing program unit

Modules, submodules, main programs, and BLOCK DATA subprograms have names
that cannot be used within their scope, so we allow those names to be
used for other entities in the scope.  This might not be entirely
conformant with the language standard, so warn about it.

Differential Revision: https://reviews.llvm.org/D139146