[Flang] Fix build failures (NFC)

Some changes introduced in https://reviews.llvm.org/D88655 cause compiler
warnings. Since in Flang warnings are treated as errors (`-Werror` is on by
default), these are in practice compiler errors (verified with clang-10 and
clang-11, gcc-10). This patches fixes these warning/failures.

Warning/error 1:
```
llvm-project/flang/lib/Semantics/check-omp-structure.cpp:107:3: error:
unannotated fall-through between switch labels [-Werror,-Wimplicit-fallthrough]
```

Warning/error 2:
```
llvm-project/flang/lib/Semantics/check-directive-structure.h:39:9: error: field
'currentDirective_' will be initialized after field 'upperCaseDirName_'
[-Werror,-Wreorder-ctor]
```

Failing buildbot:
* http://lab.llvm.org:8011/#/builders/33/builds/386