The meat of this patch is in BuildCXXMemberCalLExpr where we make it use
MarkMemberReferenced instead of marking functions referenced directly. An audit
of callers to MarkFunctionReferenced and DiagnoseUseOfDecl also caused a few
other changes:
 * don't mark functions odr-used when considering them for an initialization
   sequence. Do mark them referenced though.
 * the function nominated by the cleanup attribute should be diagnosed.
 * operator new/delete should be diagnosed when building a 'new' expression.