[lldb/win] Fix TestIRMemoryMapWindows.test when running tests in git bash

lit.util.which('link') picks up the wrong link.exe in git bash, leading
to this error:

  # command stderr:
  /usr/bin/link: extra operand '/LIBPATH:C:\\Progra....'
  Try '/usr/bin/link --help' for more information.

Instead, assume that link.exe is next to cl.exe.

Differential Revision: https://reviews.llvm.org/D109832