gccrs: nr: Improve Macro Def/Invoc mappings

This now uses find_leaf_definition to better resolve macro definitions
to their actual definitions instead of a possible import, and likewise for
invocations. This also improves the robustness and error checking for
resolving definitions.

gcc/rust/ChangeLog:

	* resolve/rust-early-name-resolver-2.0.cc (Early::insert_once): Rename...
	(Early::try_insert_once): ...to this, and improve logic.
	(Early::go): Use new API.
	(Early::visit): Likewise.
	(Early::finalize_simple_import): Likewise.
	(Early::finalize_rebind_import): Likewise.
	* resolve/rust-early-name-resolver-2.0.h: Declare the new API.