[flang] Fix semantics check for RETURN statement

The RETURN statement is allowed in functions and subroutines, but not
in main programs. It is however a common extension, which we also
implement, to allow RETURN from main programs -- we only issue a
portability warning when -pedantic or -std=f2018 are set.

This patch fixes false positives for this portability warning, where it
was triggered also when RETURN was present in functions or subroutines.

Fixexs #55080

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D124732