AST: Also respect @_originallyDefinedIn in TypeDecl::compare()

If a protocol was moved from one module to another, we need to
continue using the old module name when comparing it against
other protocols.

Author: slavapestov

lib/AST/Decl.cpp

@@ -5353,6 +5353,13 @@ int TypeDecl::compare(const TypeDecl *type1, const TypeDecl *type2) {
     // of the ABI, and so we must take care to get the correct module
     // name for the comparison.
     auto getModuleNameForOrder = [&](const TypeDecl *decl) -> StringRef {
+      // Respect @_originallyDefinedIn on the type itself, so that
+      // moving a protocol across modules does not change its
+      // position in the order.
+      auto alternateName = decl->getAlternateModuleName();
+      if (!alternateName.empty())
+        return alternateName;
+
       // This used to just call getName(), which caused accidental ABI breaks
       // when a module is imported under different aliases.
       //

test/SILGen/conformance_requirement_order_originally_defined_in.swift

@@ -0,0 +1,14 @@
+// RUN: %target-swift-emit-silgen %s -module-name main | %FileCheck %s
+
+// REQUIRES: OS=macosx
+
+@_originallyDefinedIn(module: "AnimalKit", macOS 10.10)
+@available(macOS 10.9, *)
+public protocol Ungulate {}
+
+@_originallyDefinedIn(module: "HorseKit", macOS 10.10)
+@available(macOS 10.9, *)
+public protocol Horse {}
+
+// CHECK-LABEL: sil [ossa] @$s4main39requirementOrderWithOriginallyDefinedInyyx9AnimalKit8UngulateRz05HorseI00K0RzlF : $@convention(thin) <T where T : Ungulate, T : Horse> (@in_guaranteed T) -> () {
+public func requirementOrderWithOriginallyDefinedIn<T: Ungulate & Horse>(_: T) {}