[flang][OpenMP] Fix metadirective loop semantic checks (#208844)
This is a follow-up to https://github.com/llvm/llvm-project/pull/207088.
Post-merge review feedback:
* Rename mayVariantBeSelected to MayVariantBeSelected and move it to
the shared OpenMP semantic utilities so declare-variant checking can
reuse it.
* Explain why a WHEN selector is recorded only when its modifier list
contains exactly one element.
* Diagnose loop-associated variants that end a declaration-only
program unit, such as a module with no execution part.
Additional correctness fixes:
* Handle consecutive metadirectives and modules or submodules with
contained procedures without silently dropping an earlier
loop-associated variant.
* Prevent a loop-associated variant in an interface, BLOCK construct,
conditional branch, or nested body from being checked against an
unrelated loop outside that scope.
* Avoid false or missing loop diagnostics when match_any or match_none
combines target selectors with user or construct selectors.
Assisted with codex.
llvm-project
c57f88d5 - [flang][OpenMP] Fix metadirective loop semantic checks (#208844)
