[patch] Don't let virtual calls and dynamic casts call MarkVTableUsed()

[John McCall]

> On Jan 13, 2015, at 1:42 PM, Nico Weber <thakis at chromium.org> wrote:
> Tiny update: I missed a MarkVTableUsed() call in SemaInit.cpp. After removing that, Sema::BasePathInvolvesVirtualBase() is dead, so remove that too.

Conceptually seems fine to me.  Thanks for doing this.

John.