PmCheck: Disattach COMPLETE pragma lookup from TyCons

By not attaching COMPLETE pragmas with a particular TyCon and instead
assume that every COMPLETE pragma is applicable everywhere, we can
drastically simplify the logic that tries to initialise available
COMPLETE sets of a variable during the pattern-match checking process,
as well as fixing a few bugs.

Of course, we have to make sure not to report any of the
ill-typed/unrelated COMPLETE sets, which came up in a few regression
tests.

In doing so, we fix #17207, #18277 and #14422.

There was a metric decrease in #18478 by ~20%.

Metric Decrease:
    T18478

1 files changed, 10 insertions, 3 deletions

diff --git a/testsuite/tests/pmcheck/complete_sigs/completesig04.stderr b/testsuite/tests/pmcheck/complete_sigs/completesig04.stderr
index 21a6377ba3..a114d0199e 100644
--- a/testsuite/tests/pmcheck/complete_sigs/completesig04.stderr
+++ b/testsuite/tests/pmcheck/complete_sigs/completesig04.stderr
@@ -1,4 +1,11 @@
 
-completesig04.hs:6:1: error:
-    • Cannot form a group of complete patterns from patterns ‘Just’ and ‘L’ as they match different type constructors (‘Maybe’ resp. ‘E’)
-    • In {-# COMPLETE Just, L #-}
+completesig04.hs:10:1: warning: [-Wincomplete-patterns (in -Wextra)]
+    Pattern match(es) are non-exhaustive
+    In an equation for ‘f’:
+        Patterns not matched:
+            B
+            C
+
+completesig04.hs:12:1: warning: [-Wincomplete-patterns (in -Wextra)]
+    Pattern match(es) are non-exhaustive
+    In an equation for ‘g’: Patterns not matched: Nothing