Avoid cascade type errors from error-correction-generated code

Without this commit, the testcase produces additional spurious errors:

```scala
-- [E104] Syntax Error: tests/neg/i4373.scala:10:0 -----------------------------
10 |class A2 extends _ with _ // error // error
   |^
   |class Any is not a trait

longer explanation available when compiling with `-explain`
-- [E104] Syntax Error: tests/neg/i4373.scala:11:21 ----------------------------
11 |class A3 extends Base with _ // error
   |                     ^
   |                     class Any is not a trait

longer explanation available when compiling with `-explain`
-- Error: tests/neg/i4373.scala:12:24 ------------------------------------------
12 |class A4 extends _ with Base // error
   |                        ^^^^
   |illegal trait inheritance: superclass Any does not derive from trait Base's
   superclass Object
```

These errors make absolutely no sense for a user, since they refer to code the
user didn't write, but that was generated by the compiler's error correction
mechanism. I haven't checked if we could use `Ident(nme.ERROR)` everywhere.

I have also tried using `EmptyTree` and `EmptyTree.withPos(wildcardPos)` and
`EmptyTree.clone.withPos(wildcardPos)` as fallback trees, but all of them failed
in some way.

Author: Blaisorblade

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -2287,7 +2287,8 @@ object Parsers {
     /** ConstrApp         ::=  SimpleType {ParArgumentExprs}
      */
     val constrApp = () => {
-      val t = checkWildcard(annotType())
+      // Using Ident(nme.ERROR) to avoid causing cascade errors on non-user-written code
+      val t = checkWildcard(annotType(), fallbackTree = Ident(nme.ERROR))
       if (in.token == LPAREN) parArgumentExprss(wrapNew(t))
       else t
     }