Fix tests crashing under strict pattern binding warnings

Before this commit, the tests listed below would crash when the stricter
pattern binding rules were enforced (whether using `-source future` or
now enabled by default)

  - neg/parser-stability-14.scala
  - neg/i4453.scala
  - fuzzy/CCE-aafcaa9cd2611d22f63273738d637f5bec6e7152.scala
  - fuzzy/comment3.scala

with a common stack trace:

  dotty.tools.dotc.core.Types$PreviousErrorType cannot be cast to dotty.tools.dotc.core.Types$TermRef
  dotty.tools.dotc.transform.patmat.SpaceEngine$.isIrrefutable(Space.scala:316)
  dotty.tools.dotc.typer.Checking.recur$1(Checking.scala:828)
  dotty.tools.dotc.typer.Checking.checkIrrefutable(Checking.scala:843)
  dotty.tools.dotc.typer.Checking.checkIrrefutable$(Checking.scala:775)
  dotty.tools.dotc.typer.Typer.checkIrrefutable(Typer.scala:119)
    ...

compiler/src/dotty/tools/dotc/transform/patmat/Space.scala

@@ -312,9 +312,9 @@ object SpaceEngine {
    *  @param  unapp   The unapply function tree
    */
   def isIrrefutable(unapp: tpd.Tree, argLen: Int)(using Context): Boolean = {
-    val fun1 = tpd.funPart(unapp)
-    val funRef = fun1.tpe.asInstanceOf[TermRef]
-    isIrrefutable(funRef, argLen)
+    tpd.funPart(unapp).tpe match
+      case funRef: TermRef => isIrrefutable(funRef, argLen)
+      case _: ErrorType => false
   }
 }