The TableGen parts of the CMake build are seriously broken. This fixes

one aspect of them by having them use the (annoying, if not broken)
proper library dependency model for adding the LLVMTableGen library as
a dependency. This could manifest as a link order issue in the presence
of separate LLVM / Clang source builds with CMake and a linker that
really cares about such things.

Also, add the Support dependency to llvm-tblgen itself so that it
doesn't rely on TableGen's transitive Support dependency. A parallel
change for clang-tblgen will be forthcoming.

llvm-svn: 143531