Strip file path from the -o option while creating reproduce.txt.

This is a fix for Bugzilla 28579.

The problem is that in --reproduce links the file path in -o option is
copied verbatim. When "lld @response.txt" link is run against the
extracted test case, if -o contains anything other that a plain file
name, the link will likely fail because the target directory in -o may
not exists. Stripping the directory path will create the output file
in the top level test directory.

Patch by Dmitry Mikulin!

llvm-svn: 293792