[libclang] Some changes on the linker options for libclang on a CMake build.

- Use the 'libclang.exports' file.
- Pass -Wl,-current_version
- Set install name to "@rpath"

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@194134 91177308-0d34-0410-b5e6-96231b3b80d8