A reflect newMethodOverride

[nicolasstucki]

• For the purpose of overriding we should provide some convenience method (Symbol.newMethodOverride ?) which given a method symbol in a parent takes care of generating a new symbol with the correct info and flags to override it.

Originally posted by @smarter in #15024 (review)

[smarter]

We might need to pass a list of method symbols to override instead of just one to be able to override methods from multiple parents, e.g.:

trait One:
  def foo: (Any, Int) // oneFooSym
  def foo(x: Int) = 1 // overload (so we can't just rely on the method name)
trait Two:
  def foo: (Int, Any) // twoFooSym
// Generated by newClass:
class Impl extends One with Two:
  // Generated by newMethodOverride(implSym, List(oneFooSym, twoFooSym)):
  override def foo: (Int, Int) = ???