[flang][OpenMP] Rename check-omp-metadirective.cpp (NFC). (#201159)
Both METADIRECTIVE and DECLARE VARIANT fall into the "variant
directives" category, so check-omp-variant.cpp is a more accurate name
for the file that hosts their semantic checks.
Suggested in
https://github.com/llvm/llvm-project/pull/198799#issuecomment-4576970335