message: Make SwiftASTContext::EitherComparator::operator() const.
  Our build of swift-lldb within Google is transitioning to libc++,
  which is stricter than libstdc++ about the const-ness of comparators
  used with std collections.
diff: @@ -57,7 +57,7 @@ public:
  private:
  struct EitherComparator {
  - bool operator()(const TypeOrDecl &r1, const TypeOrDecl &r2) {
  + bool operator()(const TypeOrDecl &r1, const TypeOrDecl &r2) const {
  auto r1_as1 = r1.GetAs<CompilerType>();
  auto r1_as2 = r1.GetAs<swift::Decl *>();
  