Parse box expressions

Parse box expressions

Add support for old box expression syntax.

gcc/rust/ChangeLog:

	* ast/rust-ast-collector.cc (TokenCollector::visit): Add visit member
	function for BoxExpr nodes.
	* ast/rust-ast-collector.h: Add visit function prototype.
	* ast/rust-ast-visitor.cc (DefaultASTVisitor::visit): Add visit member
	function to default ast visitor.
	* ast/rust-ast-visitor.h: Add visit function's prototype.
	* ast/rust-ast.cc (BoxExpr::as_string): Add as_string function
	implementation for BoxExpr.
	(BoxExpr::accept_vis): Add accept_vis implementation to BoxExpr.
	* ast/rust-expr.h (class BoxExpr): Add BoxExpr class to represent boxed
	expressions.
	* expand/rust-derive.h: Add BoxExpr visit function prototype.
	* hir/rust-ast-lower-base.cc (ASTLoweringBase::visit): Add BoxExpr
	visitor implementation.
	* hir/rust-ast-lower-base.h: Add visit function's prototype.
	* hir/rust-ast-lower-expr.cc (ASTLoweringExpr::visit): Add BoxExpr
	visitor implementation.
	* hir/rust-ast-lower-expr.h: Add visit function's prototype.
	* parse/rust-parse-impl.h (Parser::parse_box_expr): Add parse_box_expr
	function's implementation.
	* parse/rust-parse.h: Add parse_box_expr function's prototype.
	* resolve/rust-ast-resolve-base.cc (ResolverBase::visit): Add resolver
	visit implementation.
	* resolve/rust-ast-resolve-base.h: Add resolver's visit function
	prototype.

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