Allow specification of what kinds of class members to dump.

Previously when dumping class definitions, there were only
two modes - on or off.  But it's useful to sometimes get a
little more fine-grained.  For example, you might only want
to see the record layout (for example to look for extraneous
padding).  This patch adds a third mode, layout mode, which
does exactly that.  Only this-relative data members are
displayed in this mode.

Differential Revision: https://reviews.llvm.org/D31794

llvm-svn: 299733