Move second CheckUnused after pattern matcher

[som-snytt]

Witness equality checks introduced by patmat.
Must rewind when reporting, in particular contextual params became context functions and broke test for marker traits.

Fixes #25100

[felix-hedenstrom]

This string is incorrect. I updated the match condition but forgot to update the string when I made the issue 😅

[som-snytt]

There's a fine tradition of not altering the pristine example from the ticket!

[som-snytt]

SemanticDB specifically has a second phase to pick up diagnostics, so it must also move.

One diagnostic has gone missing. My impression that semanticdb (about which I know nothing) is just a hassle for testing is due to this experience. It's a big opaque file. Usually I just shrug and commit.

diff --git a/tests/semanticdb/metac.expect b/tests/semanticdb/metac.expect
index 76ad77ba87..4cc400d526 100644
--- a/tests/semanticdb/metac.expect
+++ b/tests/semanticdb/metac.expect
@@ -5366,7 +5366,7 @@ Text => empty
 Language => Scala
 Symbols => 50 entries
 Occurrences => 78 entries
-Diagnostics => 6 entries
+Diagnostics => 5 entries
 Synthetics => 2 entries

 Symbols:
@@ -5507,7 +5507,6 @@ Diagnostics:
 [9:42..9:43): [warning] unused explicit parameter
 [21:11..21:12): [warning] unused explicit parameter
 [24:24..24:27): [warning] unused pattern variable
-[25:27..25:28): [warning] unused pattern variable

 Synthetics:
 [23:6..23:10):List => *.unapplySeq[Nothing]

Probably it is the t which after patmat looks like it is used.

case val x6: List[t] @t = x5.$asInstanceOf[List[t] @t]

in source

??? match { case _: List[t] => ??? }

There was no test (!) so I added the "regular" test, as a TODO.