[Flang][CMake] Add explicit libFortranCommon dependency for f18 etc.

When I tried Solaris builds with `-DBUILD_SHARED_LIBS=ON`, some commands failed
to link:

  [ 94%] Linking CXX executable ../../../../bin/f18
  Undefined                       first referenced
   symbol                             in file
  Fortran::common::IntrinsicTypeDefaultKinds::set_sizeIntegerKind(int) CMakeFiles/f18.dir/f18.cpp.o  (symbol belongs to implicit dependency /var/llvm/local-amd64-release-shared-gcc8-make/lib/libFortranCommon.so.11git)
  Fortran::common::IntrinsicTypeDefaultKinds::set_subscriptIntegerKind(int) CMakeFiles/f18.dir/f18.cpp.o  (symbol belongs to implicit dependency /var/llvm/local-amd64-release-shared-gcc8-make/lib/libFortranCommon.so.11git)
  Fortran::common::EnumIndexToString[abi:cxx11](int, char const*) CMakeFiles/f18.dir/f18.cpp.o  (symbol belongs to implicit dependency /var/llvm/local-amd64-release-shared-gcc8-make/lib/libFortranCommon.so.11git)
  Fortran::common::IntrinsicTypeDefaultKinds::set_defaultIntegerKind(int) CMakeFiles/f18.dir/f18.cpp.o  (symbol belongs to implicit dependency /var/llvm/local-amd64-release-shared-gcc8-make/lib/libFortranCommon.so.11git)
  Fortran::common::IntrinsicTypeDefaultKinds::IntrinsicTypeDefaultKinds() CMakeFiles/f18.dir/f18.cpp.o  (symbol belongs to implicit dependency /var/llvm/local-amd64-release-shared-gcc8-make/lib/libFortranCommon.so.11git)
  Fortran::common::IntrinsicTypeDefaultKinds::set_defaultRealKind(int) CMakeFiles/f18.dir/f18.cpp.o  (symbol belongs to implicit dependency /var/llvm/local-amd64-release-shared-gcc8-make/lib/libFortranCommon.so.11git)
  ld: fatal: symbol referencing errors

This patch fixes this by adding explicit dependencies on `libFortranCommon`
to the affected commands.

Tested on `amd64-pc-solaris2.11`, `sparcv9-sun-solaris2.11`, and
`x86-64-pc-linux-gnu`.

Differential Revision: https://reviews.llvm.org/D78761