Take #2. Don't piggyback on the existing config.build_mode. Instead,
define a new lit feature for each build feature we need (currently
just "asserts"). Teach both autoconf'd and cmake'd Makefiles to define
this feature within test/lit.site.cfg. This doesn't require any lit
harness changes and should be more robust across build systems.
llvm-svn: 133664
67ff0718a4