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