fix(16459): add patch to syntax error

Add a patch to cover the cornercase where xml pattern in parens confuse
the parser.

Before this commit, the following code compiles,

```scala
for (case _ @ <div>empty</div> <- Seq(xml)) yield ()
```
but the following resulted in syntax error.

```scala
for (case _ @ <div>empty</div><-Seq(xml)) yield ()
```

Because `followingIsEnclosedGenerators` always comes after `for` and
`(`, I beleive it would not break the parser to early-exit when
`XMLSTART` is found.

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

@@ -928,6 +928,7 @@ object Parsers {
       lookahead.nextToken()
       while (parens != 0 && lookahead.token != EOF) {
         val token = lookahead.token
+        if (token == XMLSTART) return true
         if (token == LPAREN) parens += 1
         else if (token == RPAREN) parens -= 1
         lookahead.nextToken()

tests/run/i16459.scala

@@ -54,10 +54,12 @@ object Test {
   // Pattern match with if guard.
   // Note: This passes in Scala  2.12.18 and 2.13.12 too.
   val auxiliary8 = for (case _ @ <foo>FooBar</foo> <- Seq(xml) if true)
+    yield ()
   // Note: These pass in Scala  2.12.18 and 2.13.12.
   val auxiliary9 = for (case _ @ <foo>FooBar</foo><- Seq(xml) if true)
+    yield ()
   val auxiliary10 = for (case _ @ <foo>FooBar</foo><-Seq(xml) if true)
-   yield ()
+    yield ()
 
   }