Fix scala#3672: avoid error on type-application of ErrorType value

When a value `foo` has type `ErrorType`, `foo[Bla]` should not report an
error `value foo does not take type parameters`. Instead we propagate
the existing ErrorType.

compiler/src/dotty/tools/dotc/typer/TypeAssigner.scala

@@ -411,6 +411,8 @@ trait TypeAssigner {
           if (sameLength(argTypes, paramNames)) pt.instantiate(argTypes)
           else wrongNumberOfTypeArgs(fn.tpe, pt.typeParams, args, tree.pos)
         }
+      case err: ErrorType =>
+        err
       case _ =>
         //println(i"bad type: $fn: ${fn.symbol} / ${fn.symbol.isType} / ${fn.symbol.info}") // DEBUG
         errorType(err.takesNoParamsStr(fn, "type "), tree.pos)

compiler/test-resources/repl/errmsgs

@@ -68,3 +68,7 @@ scala> while (((  foo ))) {}
 1 | while (((  foo ))) {}
   |            ^^^
   |            not found: foo
+scala> val a: iDontExist = 1
+1 | val a: iDontExist = 1
+  |        ^^^^^^^^^^
+  |        not found: type iDontExist

tests/neg/errorTypes.scala

@@ -0,0 +1,5 @@
+object Test {
+  val x: iDontExist = 1 // error: not found: type iDontExist
+
+  val y = x.asInstanceOf[Int] // No error reported (was: value asInstanceOf does not take type parameters)
+}