gccrs: converter: Convert back Locations from spans

gccrs: converter: Convert back Locations from spans

Use spans to get locations of expanded tokens instead of using unknown
locations.

gcc/rust/ChangeLog:

	* lex/rust-lex.h: Make build_token method public.
	* lex/rust-token.h: Add a setter for a token location.
	* util/rust-token-converter.cc (convert): Add the function to
	convert from a Span to a Location.
	(from_ident): Convert Ident span to a Location.
	(from_literal): Convert Literal span to a Location.
	(from_punct): Convert Punct span to a Location.
	(from_group): Convert Group span to a Location.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>