[lldb] [lit] Set LD_LIBRARY_PATH or alike for Suite tests

[lldb] [lit] Set LD_LIBRARY_PATH or alike for Suite tests

Set LD_LIBRARY_PATH or local platform's equivalent of it when running
the 'Suite' tests.  This is necessary when running tests inside build
tree with BUILD_SHARED_LIBS enabled, in order to make the LLDB modules
load freshly built LLVM libraries.

The code is copied from clang (test/Unit/lit.cfg).  SHLIBDIR
substitution is added to site-config (already present in top-level LLDB
site-config) to future-proof this into supporting stand-alone builds
with shared LLDB libraries.

Differential Revision: https://reviews.llvm.org/D58610

llvm-svn: 354920