Remove a goofy CMake hack and use the standard CMake facilities to

Remove a goofy CMake hack and use the standard CMake facilities to
express library-level dependencies within Clang.

This is no more verbose really, and plays nicer with the rest of the
CMake facilities. It should also have no change in functionality.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@158888 91177308-0d34-0410-b5e6-96231b3b80d8