[MSVC] If unable to find link.exe from a MSVC installation, look for link.exe next to cl.exe

Previously, if the MSVC installation isn't detected properly, clang
will later just fail to execute link.exe.

This improves using clang in msvc mode on linux, where one intentionally
might not want to point clang to the MSVC installation itself (which
isn't executable as such), but where a link.exe named wine wrapper is
available in the path next to a cl.exe named wine wrapper.

Differential Revision: https://reviews.llvm.org/D60094

llvm-svn: 358778