Fix #14811 by wiring in $tcUnit#

Previously, we were skipping over `$tcUnit#` entirely when
wiring in `Typeable` tycons, resulting in #14811. Easily fixed.

Test Plan: make test TEST=T14811

Reviewers: bgamari, dfeuer

Reviewed By: dfeuer

Subscribers: dfeuer, rwbarton, thomie, carter

GHC Trac Issues: #14811

Differential Revision: https://phabricator.haskell.org/D4414

(cherry picked from commit d5ac5820)

compiler/typecheck/TcTypeable.hs

@@ -345,9 +345,8 @@ mkPrimTypeableTodos
 -- Note [Built-in syntax and the OrigNameCache] in IfaceEnv for more.
 ghcPrimTypeableTyCons :: [TyCon]
 ghcPrimTypeableTyCons = concat
-    [ [ runtimeRepTyCon, vecCountTyCon, vecElemTyCon
-      , funTyCon, tupleTyCon Unboxed 0 ]
-    , map (tupleTyCon Unboxed) [2..mAX_TUPLE_SIZE]
+    [ [ runtimeRepTyCon, vecCountTyCon, vecElemTyCon, funTyCon ]
+    , map (tupleTyCon Unboxed) [0..mAX_TUPLE_SIZE]
     , map sumTyCon [2..mAX_SUM_SIZE]
     , primTyCons
     ]

testsuite/tests/typecheck/should_compile/T14811.hs

+{-# language UnboxedTuples #-}
+module T14811 where
+
+data Foo a = Foo (# a #)
+data Bar = Bar (# #)

testsuite/tests/typecheck/should_compile/all.T

@@ -592,3 +592,4 @@ test('T14590', normal, compile, ['-fdefer-type-errors -fno-max-valid-substitutio
 test('T14273', normal, compile, ['-fdefer-type-errors -fno-max-valid-substitutions'])
 test('T14732', normal, compile, [''])
 test('T14763', normal, compile, [''])
+test('T14811', normal, compile, [''])