[Serialization] Always read opaque type underlying types in merge-module

When reading a swiftmodule that's part of the main module the compiler
should have access to all internal details. In that case, read the
underlying type to opaque types.

This was caught by `SILOptimizer/specialize_opaque_result_types2.sil`
which has a merge-module like behavior reading it sib files as input.

Author: xymus

lib/Serialization/Deserialization.cpp

@@ -3889,6 +3889,7 @@ class DeclDeserializer {
 
     auto *AFD = dyn_cast<AbstractFunctionDecl>(namingDecl);
     if (MF.getResilienceStrategy() == ResilienceStrategy::Resilient &&
+        !MF.FileContext->getParentModule()->isMainModule() &&
         AFD && AFD->getResilienceExpansion() != ResilienceExpansion::Minimal) {
       // Do not try to read the underlying type information if the function
       // is not inlinable in clients. This reflects the swiftinterface behavior