Don't treat dependent types as having a copy value representation. (#6055)

Add `Dependent` value and initializing representations for types whose
representations are unknown because they are dependent. When generating
SemIR in such cases, use a worst-case initializing representation that
both provides a destination address and also propagates a potential
result value.

Use this to fix incorrect lowering and lowering crashes for specific
functions involving generic types that don't use a copy value
representation.

In lowering, be careful to distinguish between whether the initializing
representation for the generic return type uses a return slot (which
affects whether the SemIR declaration and call have one) and whether the
initializing representation for the specific return type uses a return
slot (which affects whether the LLVM IR declaration and call have one).

toolchain/sem_ir/expr_info.cpp

@@ -293,6 +293,9 @@ auto FindReturnSlotArgForInitializer(const File& sem_ir, InstId init_id)
         }
         return sem_ir.inst_blocks().Get(call.args_id).back();
       }
+      case CARBON_KIND(ErrorInst _): {
+        return InstId::None;
+      }
       default:
         CARBON_FATAL("Initialization from unexpected inst {0}", init_untyped);
     }