gccrs: macro-expand: Keep optional references to last_{def, invoc}

gccrs: macro-expand: Keep optional references to last_{def, invoc}

This avoids a use-after-free when reaching the recursion limit.

gcc/rust/ChangeLog:

	* expand/rust-macro-expand.h: Keep optional references for last_def
	and last_invoc.
	* expand/rust-macro-expand.cc (MacroExpander::expand_invoc): Store
	optional references.
	* rust-session-manager.cc (Session::expansion): Use tl::optional's APIs
	for better checking.