[flang][driver] Add missing dependency (shared library builds, NFC)

`FlangFrontendTests` depends on libclangFrontend (it uses
DiagnosticConsumer classes from there). This patch adds the missing
dependency in CMake.

The missing dependency manifests itself only with BUILD_SHARED_LIBS=ON.
This symbol is linked in statically with libflangFrontend when
BUILD_SHARED_LIBS=OFF.