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

[pcc]

Summary

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.

[ruiu]

LGTM

tools/lld/CMakeLists.txt

if(UNIX)
   set(src "lld${CMAKE_EXECUTABLE_SUFFIX}")
 else()
   set(command copy)
   set(src "${LLVM_RUNTIME_OUTPUT_INTDIR}/lld${CMAKE_EXECUTABLE_SUFFIX}")
 endif()
 set(dst "${LLVM_RUNTIME_OUTPUT_INTDIR}/lld-link${CMAKE_EXECUTABLE_SUFFIX}")
 add_custom_command(TARGET lld POST_BUILD
   COMMAND ${CMAKE_COMMAND} -E ${command} ${src} ${dst})
+
+# FIXME: Remove once the lld-link2 alias is gone.
+set(dst2 "${LLVM_RUNTIME_OUTPUT_INTDIR}/lld-link2${CMAKE_EXECUTABLE_SUFFIX}")
+add_custom_command(TARGET lld POST_BUILD
+  COMMAND ${CMAKE_COMMAND} -E ${command} ${src} ${dst2})