Add support for building with clang-cl

We're working on supporting clang-cl as a compiler for Mozilla.  Here's a simple patch which adds support for this compiler to clang-cl.  Note that because clang-cl is a drop-in replacement for cl, there is not much else to be done besides this, but this does depend on http://llvm.org/bugs/show_bug.cgi?id=19981 which we're working on a fix for.