CMake: Unbreak separated LLVM/Clang project builds for Xcode.

The separate Xcode project generated for Clang is putting the clang
executables into the same location where the LLVM executables are
going. This is wrong, and breaks the Clang build because we try to
create clang++ and clang-cl symlinks in the wrong place and to the
wrong place.

As a stop-gap to get these builds working again, teach the symlink
generation to point into the LLVM executable directory instead.

llvm-svn: 198319