Make clang/test/lit.cfg pre-scan the RUN line looking for tool names,
and substitute fully qualified path names pointing to the build
directory.  This ensures we're testing the just-built tools.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@204493 91177308-0d34-0410-b5e6-96231b3b80d8