Remove one of the LLVM-specific CMake hacks in favor of standard CMake

Remove one of the LLVM-specific CMake hacks in favor of standard CMake
facilities.

This was only used in one place in LLVM, and was used pervasively (but
with different code!) in Clang. It has no advantages over the standard
CMake facilities and in some cases disadvantages.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@158889 91177308-0d34-0410-b5e6-96231b3b80d8