Push DeclGroup much farther throughout the compiler. Now the various

productions (except the already broken ObjC cases like @class X,Y;) in 
the parser that can produce more than one Decl return a DeclGroup instead
of a Decl, etc.

This allows elimination of the Decl::NextDeclarator field, and exposes
various clients that should look at all decls in a group, but which were
only looking at one (such as the dumper, printer, etc).  These have been
fixed.

Still TODO:

1) there are some FIXME's in the code about potentially using
DeclGroup for better location info.
2) ParseObjCAtDirectives should return a DeclGroup due to @class etc.
3) I'm not sure what is going on with StmtIterator.cpp, or if it can
   be radically simplified now.
4) I put a truly horrible hack in ParseTemplate.cpp.

I plan to bring up #3/4 on the mailing list, but don't plan to tackle
#1/2 in the short term.

llvm-svn: 68002