gccrs: lower: Add base for lowering FormatArgs nodes

gccrs: lower: Add base for lowering FormatArgs nodes

gcc/rust/ChangeLog:

	* Make-lang.in: Compile the new source file.
	* ast/rust-ast-collector.cc (TokenCollector::visit): Error out when
	visiting FormatArgs nodes.
	* resolve/rust-ast-resolve-base.cc (ResolverBase::visit): Likewise.
	* hir/rust-ast-lower-expr.cc (ASTLoweringExpr::visit): Likewise.
	* ast/rust-ast.cc (FormatArgs::get_locus): New.
	* ast/rust-builtin-ast-nodes.h: Improve FormatArgs API.
	* ast/rust-fmt.cc (Pieces::~Pieces): Cleanup.
	(Pieces::Pieces): Likewise.
	* ast/rust-fmt.h (struct Pieces): Add pieces_vector member.
	* hir/rust-ast-lower-format-args.cc: New file.
	* hir/rust-ast-lower-format-args.h: New file.