Remove DeclGroupOwningRef, since we intend for declarations to be owned

by DeclContexts (always) rather than by statements. 

DeclContext currently goes out of its way to avoid destroying any
Decls that might be owned by a DeclGroupOwningRef. However, in an
error-recovery situation, a failure in a declaration statement can
cause all of the decls in a DeclGroupOwningRef to be destroyed after
they've already be added into the DeclContext. Hence, DeclContext is
left with already-destroyed declarations, and bad things happen. This
problem was causing failures that showed up as assertions on x86 Linux
in test/Parser/objc-forcollection-neg-2.m.

llvm-svn: 64474