Add support for building the ASan instrumentation unit tests with the

just-built Clang binary, and linking them against the just-built ASan
runtime.

This is *very* brittle. I expect it will require tweaking, and I've
pro-actively disabled it on non-Unix builds and on cross-builds.

It is also currently missing dependency edges on GoogleTest header files
and a few other corner cases, but those can be fixed. This is the major
milestone of a mini-bootstrap-like build of the unittest.

llvm-svn: 159255