[test] Fix source-stability test and revert #18793

Author: lorentey

test/api-digester/source-stability.swift

@@ -6,5 +6,3 @@
 // RUN: %clang -E -P -x c %S/source-stability.swift.expected -o - | sed '/^\s*$/d' > %t.tmp/source-stability.swift.expected
 // RUN: %clang -E -P -x c %t.tmp/changes.txt -o - | sed '/^\s*$/d' > %t.tmp/changes.txt.tmp
 // RUN: diff -u %t.tmp/source-stability.swift.expected %t.tmp/changes.txt.tmp
-
-// REQUIRES: rdar_43425867

test/api-digester/source-stability.swift.expected

@@ -1,4 +1,6 @@
 /* FIXME: Bogus */
+Struct DictionaryIterator has generic signature change from <Key, Value where Key : Hashable> to <Element>
+Struct SetIterator has generic signature change from <Element where Element : Hashable> to <Key, Value>
 TypeAlias DictionaryIterator.Element has been removed
 TypeAlias SetIterator.Element has been removed
 Var DictionaryIterator.customMirror has been removed