const FieldDescriptorCompare
const FieldDescriptorCompare

Clang now validates that <set> comparators must have a const operator():
https://reviews.llvm.org/rL291969

Discussion:
https://groups.google.com/d/msg/protobuf/9W6zFIHaJ-4/9RrfwelpEQAJ
google/protobuf,google/protobuf,Livefyre/protobuf,google/protobuf,google/protobuf,google/protobuf,Livefyre/protobuf,Livefyre/protobuf,google/protobuf,Livefyre/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,Livefyre/protobuf,google/protobuf