Implement more of the inductive case for vtable layout involving

virtual base primaries and improve the layout of classes with virtual
bases.  WIP.

Hey, I've decided I want a change to FileCheck, I need to ensure that
the group is together, nothing in between.  Can we change it to check
the match line is from the line immediately following the last matched
line, if the source for the matched line is immediately after the
source for the previously matched line?

// CHECK: 1
// CHECK: 2
// CHECK: 3

// CHECK: 4
// CHECK: 5
// CHECK: 6

would require 1 2 and 3 to be continuous in the output, and 4 5 and 6
to be continuous.

llvm-svn: 78638