[test] Extend llvm_shlib_dir fix to unittests

Extend the fix from rL286952 to unittests. The fix added clang built
library directories (via llvm_shlib_dir) to LD_LIBRARY_PATH.
The previous logic has used llvm_libs_dir only which points to installed
LLVM when doing stand-alone builds.

The patch also removes the redundant win32 code that is no longer
necessary now that shlibdir is used unconditionally.

Differential Revision: https://reviews.llvm.org/D27812

llvm-svn: 289865