Print internal representation of ' and $ with -Yprint-debug

[nicolasstucki]

sbt:dotty> dotc Foo.scala -Xprint:front
result of Foo.scala after frontend:
package <empty> {
  class Foo() extends Object() { 
    def foo(i: quoted.Expr[Int]): quoted.Expr[Int] = 
      '{
        {
          val a: Int = ${i}
          a:Int
        }
      }
  }
}

sbt:dotty> dotc Foo.scala -Xprint:front -Yprint-debug
result of Foo.scala after frontend:
package <empty> {
  class Foo() extends Object.<init>() { 
    def foo(i: quoted.Expr[Int]): quoted.Expr[Int] = 
      scala.internal.Quoted.exprQuote[Int^](
        {
          val a: Int = scala.internal.Quoted.exprSplice[Int^](i)
          a:Int
        }
      )
  }
}

[liufengyun]

LGTM