[PECOFF] Rename link.exe -> lld-link.exe.

Creating the file "link.exe" made some confusion, so it's better to
name it lld-link.exe, as we did for CL (clang-cl.exe).

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@204509 91177308-0d34-0410-b5e6-96231b3b80d8