[ModuleInterface] Prefer the swiftmodule in the SDK over the prebuilt-cache

Fix the search order for module interfaces to respect the documented
behavior:

1. Local cache
2. Next to the .swiftinterface
3. Prebuilt cache

Fixing the order of the last two allows the XcodeDefault compiler to
prioritize swiftmodule files with SPI information.

rdar://problem/64097455

test/ModuleInterface/loading-order.swift

@@ -0,0 +1,41 @@
+/// Test the loading order of module interfaces between the SDK and the
+/// prebuilt cache. The order should be:
+///
+/// 1. Local cache (not tested here)
+/// 2. Next to the swiftinterface file
+/// 3. Prebuilt-module cache
+
+/// Create folders for a) our Swift module, b) the module cache, and c) a
+/// fake resource dir with a default prebuilt module cache inside.
+// RUN: %empty-directory(%t/MyModule.swiftmodule)
+// RUN: %empty-directory(%t/ModuleCache)
+// RUN: %empty-directory(%t/ResourceDir/%target-sdk-name/prebuilt-modules/MyModule.swiftmodule)
+
+/// Define two sets of public API.
+// RUN: echo 'public func nextToSwiftinterface() {}' > %t/NextToSwiftinterface.swift
+// RUN: echo 'public func prebuiltModule() {}' > %t/PrebuiltModule.swift
+
+/// Compile this into a module in the SDK.
+// RUN: %target-swift-frontend -emit-module %t/NextToSwiftinterface.swift -o %t/MyModule.swiftmodule/%target-swiftmodule-name -module-name MyModule -parse-stdlib -emit-module-interface-path %t/MyModule.swiftmodule/%target-swiftinterface-name
+
+/// Also put a module with a different API into the default prebuilt cache under the same name.
+// RUN: %target-swift-frontend -emit-module %t/PrebuiltModule.swift -o %t/ResourceDir/%target-sdk-name/prebuilt-modules/MyModule.swiftmodule/%target-swiftmodule-name -module-name MyModule -parse-stdlib
+
+/// Import this module and expect to use the swiftmodule next to the swiftinterface.
+// RUN: %target-swift-frontend -typecheck -resource-dir %t/ResourceDir -I %t %s -parse-stdlib -module-cache-path %t/ModuleCache -sdk %t -D FIRST_NEXT_TO_SWIFTINTERFACE
+
+/// Remove the first swiftmodule and import again to use the prebuilt swiftmodule.
+// RUN: rm %t/MyModule.swiftmodule/%target-swiftmodule-name
+// RUN: %target-swift-frontend -typecheck -resource-dir %t/ResourceDir -I %t %s -parse-stdlib -module-cache-path %t/ModuleCache -sdk %t -D THEN_PREBUILT_MODULE
+
+import MyModule
+
+#if FIRST_NEXT_TO_SWIFTINTERFACE
+
+nextToSwiftinterface()
+
+#elseif THEN_PREBUILT_MODULE
+
+prebuiltModule()
+
+#endif