[flang][driver] Add missing dependency in CMake files

The Flang driver depends on libclangBasic. This means, among other
things, that some of the tablegen files (e.g. diagnostic definitions)
need to be generated before various libclangBasic header files can be
included (e.g. DiagnosticIDs.h). If we are lucky, libclangBasic is
indeed built before various flang driver libraries that depend on it
are. This patch makes sure that this is deterministic - i.e.
libclangBasic is built before the Flang driver libraries are.

Differential Revision: https://reviews.llvm.org/D88110