Make sure to rebuild dependencies for the `check' and `unittests' targets so

we're not testing out-of-date code.  This also makes "make check" and
"make unittests" work out-of-the box right after the configure step, without
requiring the user to run "make tools-only" or "make libs-only".

Tested on Linux/x86_64 and Darwin/x86.

llvm-svn: 68708