gccrs: ast: Improve Fragment API

gcc/rust/ChangeLog:

	* ast/rust-ast-fragment.cc (Fragment::Fragment): Add better APIs.
	(Fragment::complete): New function.
	(Fragment::unexpanded): New function.
	* ast/rust-ast-fragment.h: Declare new APIs and add documentation.
	* expand/rust-attribute-visitor.h: Use new Fragment API.
	* expand/rust-macro-builtins.cc (MacroBuiltin::file): Likewise.
	(MacroBuiltin::column): Likewise.
	(MacroBuiltin::include_bytes): Likewise.
	(MacroBuiltin::include_str): Likewise.
	(MacroBuiltin::concat): Likewise.
	(MacroBuiltin::env): Likewise.
	(MacroBuiltin::cfg): Likewise.
	(MacroBuiltin::include): Likewise.
	(MacroBuiltin::line): Likewise.
	* expand/rust-macro-expand.cc (parse_many): Likewise.
	(transcribe_expression): Likewise.
	(transcribe_type): Likewise.
	* expand/rust-macro-expand.h (struct MacroExpander): Likewise.