Improve fidelity of ownership mapping.

The Stmt stack has been converted to a NodeRef stack. A NodeRef points to either
a Stmt or a Decl.

When emitting a call argument's Expr, push that Expr onto the node stack.

When emitting a Decl, push that Decl onto the node stack.

When emitting a Var with automatic storage duration that has an initializer,
push the init Expr onto the node stack.

MetadataInserter::attachMetadata() will now attach either a pointer for a Stmt
or a pointer for a Decl, whichever is at the back of the node stack.

Added PushDeclForScope, which is analagous to PushStmtForScope.