Revert "[CMake] Use CLANG_BINARY_DIR instead of LLVM_BINARY_DIR as installation path for Clang headers."

This was breaking some tests.  Will investigate.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@189403 91177308-0d34-0410-b5e6-96231b3b80d8