[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201062 91177308-0d34-0410-b5e6-96231b3b80d8
[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201063 91177308-0d34-0410-b5e6-96231b3b80d8