[clang-cl] Let /W4 map to -Wall -Wextra instead of just -Wall.

There's no way to make a flag alias to two flags, so add a /WCL4 flag that
maps to the All, Extra diag groups.  Fixes PR25563.

http://reviews.llvm.org/D15350


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@255382 91177308-0d34-0410-b5e6-96231b3b80d8