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

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

clang::TextDiagnosticBuffer is only reported as missing when compiling
`flang-new` with BUILD_SHARED_LIBS=ON. This symbol is linked in
statically with libflangFrontend when BUILD_SHARED_LIBS=OFF.