Make commands printed by llvm-lit include the build path in lit.cfg.

When llvm-lit prints a failure, you'll see something like 'lld *command*' However, you can't then take this, paste it in to a terminal and run it, because it's not got the absolute path of lld.

llvm and clang's lit.cfg files contain lists of commands to look for which are substituted by their full paths. So now you'd see something like '*build dir*/bin/lld *command*'.

This patch adds the same capability to lld's lit.cfg

Reviewed by Rafael Espíndola

llvm-svn: 255283