reflect: inherit all bits of flagRO in Value.Addr()

Currently, Value.Addr() collapses flagRO, which is a combination of
flagEmbedRO and flagStickyRO, to flagStickyRO. This causes exported
fields of unexported anonymous field from Value.Addr().Elem() read only.

This commit fix this by inheriting all bits of flagRO from origin
value in Value.Addr(). This should be safe due to following reasons:
* Result of Value.Addr() is not CanSet() because of it is not CanAddr()
  but not flagRO.
* Addr().Elem() get same flagRO as origin, so it should behave same as
  origin in CanSet().

Fixes #32772.

Author: kezhuw

src/reflect/value.go

@@ -257,7 +257,10 @@ func (v Value) Addr() Value {
 	if v.flag&flagAddr == 0 {
 		panic("reflect.Value.Addr of unaddressable value")
 	}
-	return Value{v.typ.ptrTo(), v.ptr, v.flag.ro() | flag(Ptr)}
+	// Inherits possible read only flags from this value,
+	// so that later Elem() will restore equivalent value back.
+	fl := v.flag & flagRO
+	return Value{v.typ.ptrTo(), v.ptr, fl | flag(Ptr)}
 }
 
 // Bool returns v's underlying value.