And an overload for ValidateObjCClassPrefixes().

This version defaults all the options, so folks don't have to pass it.

Since Options never had a PROTOC_EXPORT, it isn't immediately clear how well the
existing api worked for any shared lib cases, so this should give a simpler api
for the basic usage (if anyone wants to use it).

src/google/protobuf/compiler/objectivec/objectivec_helpers.cc

@@ -1454,6 +1454,13 @@ bool ValidateObjCClassPrefix(
 
 }  // namespace
 
+bool ValidateObjCClassPrefixes(const std::vector<const FileDescriptor*>& files,
+                               std::string* out_error) {
+    // Options's ctor load from the environment.
+    Options options;
+    return ValidateObjCClassPrefixes(files, options, out_error);
+}
+
 bool ValidateObjCClassPrefixes(const std::vector<const FileDescriptor*>& files,
                                const Options& generation_options,
                                std::string* out_error) {

src/google/protobuf/compiler/objectivec/objectivec_helpers.h

@@ -253,7 +253,11 @@ IsProtobufLibraryBundledProtoFile(const FileDescriptor* file);
 // and the result is false.
 bool PROTOC_EXPORT ValidateObjCClassPrefixes(
     const std::vector<const FileDescriptor*>& files,
-    const Options& generation_options, std::string* out_error);
+    const Options& validation_options, std::string* out_error);
+// Same was the other ValidateObjCClassPrefixes() calls, but the options all
+// come from the environment variables.
+bool PROTOC_EXPORT ValidateObjCClassPrefixes(
+    const std::vector<const FileDescriptor*>& files, std::string* out_error);
 
 // Generate decode data needed for ObjC's GPBDecodeTextFormatName() to transform
 // the input into the expected output.