cmake: create a relative symlink to clang, not absolute

Hardcoding the location of the build directory seems like it would
break at `make install`/packaging time.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@150201 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed