[Serialization] Soft-reject swiftmodules built against a different SDK

Change the way swiftmodules built against a different SDK than their
clients are rejected. This makes them silently ignored when the module
can be rebuilt from their swiftinterface, instead of reporting a hard
error.

rdar://93257769

unittests/FrontendTool/ModuleLoadingTests.cpp

@@ -149,7 +149,7 @@ protected:
 
     auto bufData = (*bufOrErr)->getBuffer();
     auto validationInfo = serialization::validateSerializedAST(
-        bufData, silOpts.EnableOSSAModules);
+        bufData, silOpts.EnableOSSAModules, /*requiredSDK*/StringRef());
     ASSERT_EQ(serialization::Status::Valid, validationInfo.status);
     ASSERT_EQ(bufData, moduleBuffer->getBuffer());
   }