gccrs: Add a default AST visitor

gccrs: Add a default AST visitor

This will allow us to derive other visitors from it and overload only a
few selected visit methods.

gcc/rust/ChangeLog:

	* Make-lang.in: Add the new visitor object file.
	* ast/rust-ast-visitor.h (class DefaultASTVisitor): Create the default
	visitor class.
	* ast/rust-ast.h: Add a new reference getter for visitor pattern.
	* ast/rust-ast-visitor.cc: New file.

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