Handling of attributes for struct / union variables

(* nowrshmsk *) on a struct / union variable now affects dynamic
bit slice assignments to members of the struct / union.

(* nowrshmsk *) can in some cases yield significant resource savings; the
combination of pipeline shifting and indexed writes is an example of this.

Constructs similar to the one below can benefit from (* nowrshmsk *), and
in addition it is no longer necessary to split out the shift assignments
on separate lines in order to avoid the error message "ERROR: incompatible
mix of lookahead and non-lookahead IDs in LHS expression."

    always_ff @(posedge clk) begin
        if (rotate) begin
            { v5, v4, v3, v2, v1, v0 } <= { v4, v3, v2, v1, v0, v5 };

            if (res) begin
                v0.bytes <= '0;
            end else if (w) begin
                v0.bytes[addr] <= data;
            end
        end
    end

frontends/verilog/verilog_parser.y

@@ -1809,7 +1809,12 @@ enum_decl: enum_type enum_var_list ';'		{ delete $1; }
 // struct or union
 //////////////////
 
-struct_decl: struct_type struct_var_list ';' 	{ delete astbuf2; }
+struct_decl:
+	attr struct_type {
+		append_attr($2, $1);
+	} struct_var_list ';' {
+		delete astbuf2;
+	}
 	;
 
 struct_type: struct_union { astbuf2 = $1; } struct_body { $$ = astbuf2; }