verilog: fix multiple AST_PREFIX scope resolution issues

- Root AST_PREFIX nodes are now subject to genblk expansion to allow
  them to refer to a locally-visible generate block
- Part selects on AST_PREFIX member leafs can now refer to generate
  block items (previously would not resolve and raise an error)
- Add source location information to AST_PREFIX nodes

frontends/verilog/verilog_parser.y

@@ -2973,6 +2973,7 @@ rvalue:
 	hierarchical_id '[' expr ']' '.' rvalue {
 		$$ = new AstNode(AST_PREFIX, $3, $6);
 		$$->str = *$1;
+		SET_AST_NODE_LOC($$, @1, @6);
 		delete $1;
 	} |
 	hierarchical_id range {