Fix ValDef span assignment in PatternMatcher

[Florian3k]

Fixes #15535

Problem was in ValDef creation in PatternMatcher.
When the following code

def m(x: Int): Unit = 
  {
    x match 
      {
        case y @ _ => 
          println(y)
          println(y)
      }
  }

gets transformed to

def m(x: Int): Unit = 
  {
    matchResult1[Unit]: 
      {
        case val x1: (x : Int) = x
        case val y: Int = x1
        return[matchResult1] 
          {
            println(y)
            println(y)
          }
      }
  }

there are new ValDefs created for x1 and y. Their spans are assigned by Positioned.envelope based on spans of ValDef's fields. Those spans not always exist. In that case, span is assigned in seq function based on span of the block, which is usually incorrect.

Solution is to set ValDef's span to the span of original TermSymbol from which current val is being created.