Support tracing of struct member accessor methods

This follows the same approach used for attr_reader/attr_writer in
2d98593bf5, skipping the checking for
tracing after the first call using the call cache, and clearing the
call cache when tracing is turned on/off.

Fixes [Bug #18886]

test/ruby/test_settracefunc.rb

@@ -955,6 +955,55 @@ CODE
     assert_equal(expected*2, events)
   end
 
+  def test_tracepoint_struct
+    c = Struct.new(:x) do
+      alias y x
+      alias y= x=
+    end
+    obj = c.new
+
+    ar_meth = obj.method(:x)
+    aw_meth = obj.method(:x=)
+    aar_meth = obj.method(:y)
+    aaw_meth = obj.method(:y=)
+    events = []
+    trace = TracePoint.new(:c_call, :c_return){|tp|
+      next if !target_thread?
+      next if tp.path != __FILE__
+      next if tp.method_id == :call
+      case tp.event
+      when :c_call
+        assert_raise(RuntimeError) {tp.return_value}
+        events << [tp.event, tp.method_id, tp.callee_id]
+      when :c_return
+        events << [tp.event, tp.method_id, tp.callee_id, tp.return_value]
+      end
+    }
+    test_proc = proc do
+      obj.x = 1
+      obj.x
+      obj.y = 2
+      obj.y
+      aw_meth.call(1)
+      ar_meth.call
+      aaw_meth.call(2)
+      aar_meth.call
+    end
+    test_proc.call # populate call caches
+    trace.enable(&test_proc)
+    expected = [
+      [:c_call, :x=, :x=],
+      [:c_return, :x=, :x=, 1],
+      [:c_call, :x, :x],
+      [:c_return, :x, :x, 1],
+      [:c_call, :x=, :y=],
+      [:c_return, :x=, :y=, 2],
+      [:c_call, :x, :y],
+      [:c_return, :x, :y, 2],
+    ]
+    assert_equal(expected*2, events)
+  end
+
   class XYZZYException < Exception; end
   def method_test_tracepoint_raised_exception err
     raise err