[flang] Warn about construct names that are not distinct in the inclusive scope

f18 implements BLOCK scoping for construct names, like most but not all Fortran
compilers, but in the 2018 standard such names are defined to be local identifiers
whose scope is the inclusive scope -- i.e., the subprogram or main program.
Detect usage that depends on this extension and emit a portability warning.

Differential Revision: https://reviews.llvm.org/D143776