[cmake] Fix -rpath-link in stand-alone build

Set LLVM_LIBRARY_OUTPUT_INTDIR as expected by llvm_setup_rpath() macro
when doing stand-alone builds. This is required to pass correct
-rpath-link when linking shared libraries, and therefore ensure that
the linker can find dependency libraries correctly during the build.

Differential Revision: https://reviews.llvm.org/D29099

llvm-svn: 293078