New round of fixes for "Always compile debuginfo-tests for the host triple"

clang tests were breaking, at least when compiling clang only, from an
installed llvm. Make the lit.cfg script deal with the case when we don't
have a host_triple available.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@220148 91177308-0d34-0410-b5e6-96231b3b80d8