[lit] Set shlibpath_var on Solaris

Summary:
During make check-all on Solaris, lit complains

llvm-lit: /vol/gcc/src/llvm/llvm/dist/tools/clang/test/Unit/lit.cfg.py:57: warning: unable to inject shared library path on 'SunOS'

The following patch avoids this: Solaris uses LD_LIBRARY_PATH like several other targets.

In theory, one could also handle LD_LIBRARY_PATH_{32,64} which take precedence over
LD_LIBRARY_PATH if set, but let's cross that bridge when we get there.

Patch by Rainer Orth.

Reviewers: rsmith, lichray
Reviewed By: lichray

Differential Revision: https://reviews.llvm.org/D39640

llvm-svn: 319026