lit support for REQUIRES: asserts.

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.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@133665 91177308-0d34-0410-b5e6-96231b3b80d8