Add regression test for #18755.

Close #18755

testsuite/tests/ghci/scripts/T18755.script

+:set -fprint-explicit-runtime-reps
+:t ($)

testsuite/tests/ghci/scripts/T18755.stdout

+($)
+  :: forall (r :: GHC.Types.RuntimeRep) a (b :: TYPE r).
+     (a -> b) -> a -> b

testsuite/tests/ghci/scripts/all.T

@@ -320,3 +320,4 @@ test('T17549', normal, ghci_script, ['T17549.script'])
 test('T17669', [extra_run_opts('-fexternal-interpreter -fobject-code'), expect_broken(17669)], ghci_script, ['T17669.script'])
 test('T18501', normal, ghci_script, ['T18501.script'])
 test('T18644', normal, ghci_script, ['T18644.script'])
+test('T18755', normal, ghci_script, ['T18755.script'])