Added dependency on the TableGen executable file.

For the case when LLVM_OPTIMIZED_TABLEGEN is ON (enables LLVM_USE_HOST_TOOLS),
we need both _TABLEGEN_TARGET and _TABLEGEN_EXE in the  DEPENDS list
to have .inc files rebuilt on a tablegen change, as cmake does not propagate
file-level dependencies of custom targets.

We could always have just one dependency on both the target and
the file, but the 2 cases would produce cleaner cmake files.

llvm-svn: 305142