[PATCH] Allow client use of -Wimplicit-fallthrough and fix a potential fallthrough bug

[Tim Northover]

> I attempted to build the rest of Clang with the warning turned on.
> Most of the uncommented fallthroughs look benign, save for one in
> lib/Driver/ToolChains.cpp; I don't think aarch64 should fall through
> to check for mips libraries if aarch64 isn't available.

Oh, well spotted! I've also committed this one as r183956.

Thanks.

Tim.