Separate pattern types from expression types (#5360)

This is a step toward treating patterns as compile-time constants, so
that we can import them more easily.

toolchain/sem_ir/function.cpp

@@ -95,7 +95,8 @@ auto Function::GetDeclaredReturnType(const File& file,
   if (!return_slot_pattern_id.has_value()) {
     return TypeId::None;
   }
-  return GetTypeOfInstInSpecific(file, specific_id, return_slot_pattern_id);
+  return ExtractScrutineeType(
+      file, GetTypeOfInstInSpecific(file, specific_id, return_slot_pattern_id));
 }
 
 }  // namespace Carbon::SemIR