Add clang-cl alias to allow users to disable c4005

If we ever want three or more aliases, at that point we should put MSVC
warning ids in DiagnosticGroups.td.  We can use that to support #pragma
warning.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@205598 91177308-0d34-0410-b5e6-96231b3b80d8