Implement lowering ReferencePattern from AST to HIR

Implement lowering ReferencePattern from AST to HIR

gcc/rust/ChangeLog:

	* ast/rust-pattern.h:
	(ReferencePattern::is_double_reference): Add method.
	(ReferencePattern::get_is_mut): Add method.
	* hir/rust-ast-lower-pattern.cc
	(ASTLoweringPattern::visit): Add ReferencePattern visitor.
	* hir/rust-ast-lower-pattern.h:
	(ASTLoweringPattern::visit): Add ReferencePattern visitor.

Signed-off-by: Owen Avery <powerboat9.gamer@gmail.com>