[Serialization] Serialize Clang types for non-modular headers.

Author: varungandhi-apple

lib/ClangImporter/Serializability.cpp

@@ -53,8 +53,10 @@ class SerializationPathFinder {
           Impl.SwiftContext.getSwiftDeclForExportedClangDecl(decl))
       return swiftDecl;
 
-    // Otherwise we have no way to find it.
-    return StableSerializationPath();
+    // Allow serialization for non-modular headers as well, with the hope that
+    // we find the same header when doing unqualified lookup during
+    // deserialization.
+    return findImportedPath(named);
   }
 
 private:

test/ClangImporter/unserializable-clang-function-types.swift

@@ -1,5 +1,6 @@
 // RUN: %empty-directory(%t)
-// RUN: not --crash %target-swift-frontend(mock-sdk: %clang-importer-sdk) %s -enable-testing -module-name NonModularApp -emit-module -o %t/NonModularApp.swiftmodule -import-objc-header %S/Inputs/non-modular-header.h -DNON_MODULAR_APP -use-clang-function-types 2>&1 | %FileCheck %s
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) %s -enable-testing -module-name NonModularApp -emit-module -o %t/NonModularApp.swiftmodule -import-objc-header %S/Inputs/non-modular-header.h -DNON_MODULAR_APP -use-clang-function-types
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) %s -typecheck -module-name NonModularAppTest -I %t -DNON_MODULAR_APP_TEST
 
 // CHECK: Clang function type is not serializable
 
@@ -9,3 +10,9 @@ struct S {
   static func f(_ : @convention(c, cType: "void (*)(PlaceholderType, size_t)") (PlaceholderType, Int) -> ()) {}
 }
 #endif
+
+#if NON_MODULAR_APP_TEST
+@testable import NonModularApp
+
+S.f({ _, _ in })
+#endif