DR1672, DR1813, DR1881, DR2120: Implement recent fixes to "standard

DR1672, DR1813, DR1881, DR2120: Implement recent fixes to "standard
layout" rules.

The new rules say that a standard-layout struct has its first non-static
data member and all base classes at offset 0, and consider a class to
not be standard-layout if that would result in multiple subobjects of a
single type having the same address.

We track "is C++11 standard-layout class" separately from "is
standard-layout class" so that the ABIs that need this information can
still use it.

Differential Revision: https://reviews.llvm.org/D45176

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@329332 91177308-0d34-0410-b5e6-96231b3b80d8