Disallow open modifier on objects (scala#21922)

Fixes scala#21760

Author: hamzaremmal

compiler/src/dotty/tools/dotc/typer/Checking.scala

@@ -611,6 +611,8 @@ object Checking {
     val mods = mdef.mods
     def flagSourcePos(flag: FlagSet) =
       mods.mods.find(_.flags == flag).getOrElse(mdef).srcPos
+    if mods.is(Open) then
+      report.error(ModifierNotAllowedForDefinition(Open), flagSourcePos(Open))
     if mods.is(Abstract) then
       report.error(ModifierNotAllowedForDefinition(Abstract), flagSourcePos(Abstract))
     if mods.is(Sealed) then

tests/neg/i21760.scala

@@ -0,0 +1 @@
+open object O // error