Create lld-link2 symlink/copy for new COFF linker.

This situation will only be temporary; I imagine we will eventually want to
have the new linker take the lld-link alias after we remove the old one.
However, I would like to port the CFI test suite in the compiler-rt
repository to Windows using the new COFF linker's LTO support; these tests
currently use gcc-style command lines and invoke the linker via the
compiler driver.

We can select the linker using the -fuse-ld flag, which Clang supports on
Windows, but it takes an executable name, rather than a list of arguments
(so "-fuse-ld=lld -flavor link2" wouldn't work), and it doesn't seem worth
the effort to extend the Clang driver to support arguments.

Differential Revision: http://reviews.llvm.org/D11031

llvm-svn: 241696