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.

llvm-svn: 220152
GitOrigin-RevId: 090a237f2981d7059073439f4fe159cbd7ab3389
11 files changed