gccrs: rust: Replace uses of ASTFragment -> Fragment

gccrs: rust: Replace uses of ASTFragment -> Fragment

gcc/rust/ChangeLog:

	* ast/rust-ast.h (class ASTFragment): Remove old ASTFragment class.
	* ast/rust-macro.h (class MacroRulesDefinition): Use new Fragment API.
	* expand/rust-attribute-visitor.h: Likewise.
	* expand/rust-macro-builtins.cc (macro_end_token): Likewise.
	(MacroBuiltin::assert): Likewise.
	(MacroBuiltin::file): Likewise.
	(MacroBuiltin::column): Likewise.
	(MacroBuiltin::include_bytes): Likewise.
	(MacroBuiltin::include_str): Likewise.
	(MacroBuiltin::compile_error): Likewise.
	(MacroBuiltin::concat): Likewise.
	(MacroBuiltin::env): Likewise.
	(MacroBuiltin::cfg): Likewise.
	(MacroBuiltin::include): Likewise.
	(MacroBuiltin::line): Likewise.
	* expand/rust-macro-builtins.h: Likewise.
	* expand/rust-macro-expand.cc (MacroExpander::expand_decl_macro): Likewise.
	(MacroExpander::expand_invoc): Likewise.
	(MacroExpander::match_repetition): Likewise.
	(parse_many): Likewise.
	(transcribe_many_items): Likewise.
	(transcribe_many_ext): Likewise.
	(transcribe_many_trait_items): Likewise.
	(transcribe_many_impl_items): Likewise.
	(transcribe_many_trait_impl_items): Likewise.
	(transcribe_expression): Likewise.
	(transcribe_type): Likewise.
	(transcribe_on_delimiter): Likewise.
	(tokens_to_str): Likewise.
	* expand/rust-macro-expand.h (struct MacroExpander): Likewise.
	* util/rust-hir-map.cc (Mappings::insert_macro_def): Likewise.