gccrs: Add builder for current-scope drop cleanup

Add CompileDrop::build_current_scope_drop_cleanup to build the drop
calls for the current scope as one statement tree.

This allows a follow-up patch to place the same cleanup tree in a
TRY_FINALLY_EXPR.

gcc/rust/ChangeLog:

	* backend/rust-compile-drop.cc
	(CompileDrop::build_current_scope_drop_cleanup): New function to
	build current scope drop calls as a statement tree.
	(CompileDrop::emit_current_scope_drop_calls): Use it.
	* backend/rust-compile-drop.h
	(CompileDrop::build_current_scope_drop_cleanup): Declare.

Signed-off-by: Lishin <lishin1008@gmail.com>