[CMake] Re-apply r200765, "Get rid of llvm_config() to expand dependencies."

CMake's target_link_libraries() will manage dependencies with Brad's LLVMConfig improvements.

Configuration time may be reduced by a few seconds.
[CMake] LLVMProcessSources.cmake: Prune add_file_dependencies to ${TABLEGEN_OUTPUT}.

I am sure it'd not be required any more.
In trunk, all of tablegen's users depend on ${TABLEGEN_OUTPUT} as not file dependency but inter-target dependency.