Fix #9266: Offer rewrites for parentheses in implicit closures

I don't have the time to write a proper test for this. I verified manually
that it does what it should:
```
-- Migration Warning: i9266.scala:3:23 -----------------------------------------
3 |def test = { implicit x: Int => x + x }
  |                       ^
  |This syntax is no longer supported; parameter needs to be enclosed in (...)
  |This construct can be rewritten automatically under -rewrite.
1 warning found
```

Author: odersky

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

@@ -2169,11 +2169,11 @@ object Parsers {
                 // Don't error in non-strict mode, as the alternative syntax "implicit (x: T) => ... "
                 // is not supported by Scala2.x
               report.errorOrMigrationWarning(
-                s"This syntax is no longer supported; parameter needs to be enclosed in (...)",
+                s"This syntax is no longer supported; parameter needs to be enclosed in (...)${rewriteNotice()}",
                 in.sourcePos())
             in.nextToken()
             val t = infixType()
-            if (false && migrateTo3) {
+            if (sourceVersion == `3.1-migration`) {
               patch(source, Span(start), "(")
               patch(source, Span(in.lastOffset), ")")
             }

tests/pos/i9266.scala

@@ -0,0 +1,3 @@
+import language.`3.1-migration`
+
+def test = { implicit x: Int => x + x }