Make -fdump-vtable-layouts print to stdout, not stderr

This makes it consistent with -fdump-record-layouts, which was moved to
outs() in r186219.  My reasoning for going with stdout is that when one
of these options is present, the layouts are really a program output,
and shouldn't be interleaved with diagnostics, which are on stderr.

Reviewers: timurrrr

Differential Revision: http://llvm-reviews.chandlerc.com/D2127