Revert "[CMake] Fix an issue building out-of-tree introduced in r272200"

This reverts r272275. This actually wasn't the right way to fix the problem. The correct solution is in r272279.

Applying the fix to LLVM as done in r272279, means this fix will get picked up by all projects building out of tree using LLVM's CMake modules. As opposed to the fix I had in r272275, which would require each project to change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@272280 91177308-0d34-0410-b5e6-96231b3b80d8