[Clang][OpenMP] Fix -Wcovered-switch-default in CGOpenMPRuntime.cpp (NFC)

/data/llvm-project/clang/lib/CodeGen/CGOpenMPRuntime.cpp:1621:3: error: default label in switch which covers all enumeration values [-Werror,-Wcovered-switch-default]
  default:
  ^
/data/llvm-project/clang/lib/CodeGen/CGOpenMPRuntime.cpp:1643:3: error: default label in switch which covers all enumeration values [-Werror,-Wcovered-switch-default]
  default:
  ^
2 errors generated.