Invent given pattern name in for comprehension

[som-snytt]

Fixes #23119

A given pattern in a for comprehension results in a fresh val in the body of the mapping function, but it should have the same (arbitrary) name as in the rest of the expansion.

This commit gives the given its usual given name (in makeIdPat) so that the unused check can check it.

Currently, without -preview, showing that $1$ is called given_Int by typer:

➜  scala-cli repl --server=false -S 3.7.2-RC1 -Vprint:typer,refchecks -Wunused:all
Welcome to Scala 3.7.2-RC1 (17.0.15, Java OpenJDK 64-Bit Server VM).
Type in expressions for evaluation. Or try :help.

scala> for given Int <- 1 to 2; j: Int = summon[Int] yield j
1 warning found
[[syntax trees at end of                     typer]] // rs$line$1
package <empty> {
  final lazy module val rs$line$1: rs$line$1 = new rs$line$1()
  final module class rs$line$1() extends Object() { this: rs$line$1.type =>
    val res0: IndexedSeq[Int] =
      intWrapper(1).to(2).map[(Int, Int)]((x$1: Int) =>
        x$1:Int @unchecked match
          {
            case given $1$ @ _:Int =>
              val j: Int = $1$
              Tuple2.apply[Int, Int]($1$, j)
          }
      ).map[Int]((x$1: (Int, Int)) =>
        x$1:(x$1 : (Int, Int)) @unchecked match
          {
            case Tuple2.unapply[Int, Int](given given_Int @ _:Int, j @ _:Int)
               => j:Int
          }
      )
  }
}

Without -preview, this commit:

scala> for given Int <- 1 to 2; j: Int = summon[Int] yield j
[[syntax trees at end of                     typer]] // rs$line$1
package <empty> {
  final lazy module val rs$line$1: rs$line$1 = new rs$line$1()
  final module class rs$line$1() extends Object() { this: rs$line$1.type =>
    val res0: IndexedSeq[Int] =
      intWrapper(1).to(2).map[(Int, Int)]((x$1: Int) =>
        x$1:Int @unchecked match
          {
            case given given_Int @ _:Int =>
              val j: Int = given_Int
              Tuple2.apply[Int, Int](given_Int, j)
          }
      ).map[Int]((x$1: (Int, Int)) =>
        x$1:(x$1 : (Int, Int)) @unchecked match
          {
            case Tuple2.unapply[Int, Int](given given_Int @ _:Int, j @ _:Int)
               => j:Int
          }
      )
  }
}

With -preview, the tupling map is eliminated early:

scala> for given Int <- 1 to 2; j: Int = summon[Int] yield j
[[syntax trees at end of                     typer]] // rs$line$1
package <empty> {
  final lazy module val rs$line$1: rs$line$1 = new rs$line$1()
  final module class rs$line$1() extends Object() { this: rs$line$1.type =>
    val res0: IndexedSeq[Int] =
      intWrapper(1).to(2).map[Int]((x$1: Int) =>
        x$1:Int @unchecked match
          {
            case given given_Int @ _:Int =>
              val j: Int = given_Int
              j:Int
          }
      )
  }
}

[som-snytt]

@sjrd Minimized commit!