[Driver] Create "link[.exe]" symlink for the Windows driver.

Clang creates "clang-cl" as a symlink to (or a copy of) "clang" for the MSVC-
compatible driver. This patch is to do the same thing for "link" and "lld".

Differential Revision: http://llvm-reviews.chandlerc.com/D3066

llvm-svn: 203751