TyperState#commit: Flush reporter before committing

Flushing the reporter can force messages (apparently because of
`HideNonSensicalMessages#isHidden`) which can affect the TyperState
we're in the process of committing, so make sure that flushing happens
before committing to not trigger any assertion.

Fixes scala#12736

compiler/src/dotty/tools/dotc/core/TyperState.scala

@@ -140,6 +140,7 @@ class TyperState() {
     Stats.record("typerState.commit")
     assert(isCommittable, s"$this is not committable")
     assert(!isCommitted, s"$this is already committed")
+    reporter.flush()
     setCommittable(false)
     val targetState = ctx.typerState
     assert(!targetState.isCommitted, s"Attempt to commit $this into already committed $targetState")
@@ -152,7 +153,6 @@ class TyperState() {
       else
         targetState.mergeConstraintWith(this)
     targetState.gc()
-    reporter.flush()
     isCommitted = true
   }
 

tests/neg/i12736b.scala

@@ -0,0 +1,6 @@
+object Test {
+  def apply[S](r: Any)(using DoesntExist): Any = r // error
+
+  def test(o: Option[Any]) =
+    o.map(x => Test(doesntExist, x)) // error
+}