[flang][OpenMP] Fix build break with -Werror
llvm-project/flang/lib/Semantics/resolve-directives.cpp:753:7: error: default label in switch which covers all enumeration values [-Werror,-Wcovered-switch-default]
753 | default:
| ^
1 error generated.
https://lab.llvm.org/buildbot/#/builders/53/builds/17917
