`definitions_required` -> `definitions_required_by_decl` in toolchain/check/check_unit.cpp (#5329)

Updates message to reflect changes in #5090 .

---------

Co-authored-by: Josh L <josh11b@users.noreply.github.com>

toolchain/check/check_unit.cpp

@@ -477,7 +477,8 @@ auto CheckUnit::CheckRequiredDefinitions() -> void {
         CARBON_FATAL("TODO: Support interfaces in DiagnoseMissingDefinitions");
       }
       default: {
-        CARBON_FATAL("Unexpected inst in definitions_required: {0}", decl_inst);
+        CARBON_FATAL("Unexpected inst in definitions_required_by_decl: {0}",
+                     decl_inst);
       }
     }
   }