Fix 2 stage build on some apple bots.

The recent lit refactor changed the location of the lit script
run by check targets from <source>/utils/lit/lit.py to
<bin>/llvm-lit.py.  In some 2-stage build scenarios, the location
of <bin> was not properly passed through to the second stage,
and it was looking for /llvm-lit.py instead, causing failures.

Fix suggested by Mike Edwards and Chris Bieneman @apple

llvm-svn: 313300