Revert r269717. That change alone did not provide the intended benefit (which

would come from changing the type of ASTContext::DeclAttrs from
DenseMap<Decl*,AttrVec*> to DenseMap<Decl*,AttrVec>), and it turns out to be
impractical to avoid the allocation there, because we expose the address of the
attribute vector in ways that are hard to fix.

llvm-svn: 269849