Support the -Wassign-enum compiler flag. (#2085)

Support the -Wassign-enum compiler flag.
diff --git a/objectivec/ProtocolBuffers_iOS.xcodeproj/project.pbxproj b/objectivec/ProtocolBuffers_iOS.xcodeproj/project.pbxproj
index b87dea2..b6bc95a 100644
--- a/objectivec/ProtocolBuffers_iOS.xcodeproj/project.pbxproj
+++ b/objectivec/ProtocolBuffers_iOS.xcodeproj/project.pbxproj
@@ -944,6 +944,7 @@
 				CLANG_ANALYZER_SECURITY_INSECUREAPI_RAND = YES;
 				CLANG_ANALYZER_SECURITY_INSECUREAPI_STRCPY = YES;
 				CLANG_STATIC_ANALYZER_MODE = deep;
+				CLANG_WARN_ASSIGN_ENUM = YES;
 				CLANG_WARN_DEPRECATED_OBJC_IMPLEMENTATIONS = YES;
 				CLANG_WARN_DOCUMENTATION_COMMENTS = YES;
 				CLANG_WARN_NULLABLE_TO_NONNULL_CONVERSION = YES;
@@ -1005,6 +1006,7 @@
 				CLANG_ANALYZER_SECURITY_INSECUREAPI_RAND = YES;
 				CLANG_ANALYZER_SECURITY_INSECUREAPI_STRCPY = YES;
 				CLANG_STATIC_ANALYZER_MODE = deep;
+				CLANG_WARN_ASSIGN_ENUM = YES;
 				CLANG_WARN_DEPRECATED_OBJC_IMPLEMENTATIONS = YES;
 				CLANG_WARN_DOCUMENTATION_COMMENTS = YES;
 				CLANG_WARN_NULLABLE_TO_NONNULL_CONVERSION = YES;