gccrs: format-args: Fix Rust interface and add input parsing.

gcc/rust/ChangeLog:

	* ast/rust-ast.cc: Make FormatArgs inherit from AST::Expr
	* ast/rust-builtin-ast-nodes.h: Improve FormatArg* nodes and helpers.
	* ast/rust-fmt.cc (Pieces::collect): Fix interface to match FFI function.
	* ast/rust-fmt.h (collect_pieces): Likewise.
	(struct Pieces): Add append_newline parameter.
	* expand/rust-macro-builtins.cc: Add proper parsing of format_args
	input.
	* hir/rust-ast-lower-base.cc: Include diagnostics header.

libgrust/ChangeLog:

	* libformat_parser/src/lib.rs: Switch interface to use more parser
	parameters.
	* libformat_parser/src/bin.rs: Use new interface.