public String getVisitorFileName(SourceType sourceType) {
		assert gen.g.name != null;
		ST extST = getTemplates().getInstanceOf("codeFileExtension");
		String listenerName = gen.g.name + "Visitor";
		return listenerName+extST.render();
	}
A given grammar T, return the visitor name such as
 TVisitor.java, if we're using the Java target.
tool/src/org/antlr/v4/codegen/Target.java