Use X macros to generate DefaultASTVisitor

This should help us reduce mismatches between DefaultASTVisitor and
ExpandVisitor later.

clang-format is disabled for "ast/rust-ast-visitable-inner.h", as it
doesn't seem to handle the macro shenanagins used very well.

gcc/rust/ChangeLog:

	* ast/rust-ast-visitor.cc (DefaultASTVisitor::visit): Remove
	explicit definitions, use X macros.
	* ast/rust-ast-visitable-inner.h: New file.

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

Author: powerboat9