Merge pull request swiftlang#21257 from nkcsgexi/abi-checker-enhance

swift-module-digester: non-final methods in fixed-layout classes no longer have fixed orders.

Author: nkcsgexi

test/api-digester/Inputs/cake.swift

@@ -96,3 +96,7 @@ infix operator ..*..
 class UsableFromInlineClass {
   private var Prop = 1
 }
+
+class InternalType {}
+
+extension InternalType {}