Fix YAML traits for sequence types (NFC)

[eugenis]

Summary

The type might not have begin() and end() methods.

include/llvm/Support/YAMLTraits.h

void mapOptional(const char *Key, T &Val, const T &Default) {
     EmptyContext Ctx;
     mapOptionalWithContext(Key, Val, Default, Ctx);
   }
 
   template <typename T, typename Context>
   typename std::enable_if<has_SequenceTraits<T>::value, void>::type
   mapOptionalWithContext(const char *Key, T &Val, Context &Ctx) {
     // omit key/value instead of outputting empty sequence
-    if (this->canElideEmptySequence() && !(Val.begin() != Val.end()))
+    if (this->canElideEmptySequence() &&
+        SequenceTraits<T>::size(*this, Val) == 0)
       return;
     this->processKey(Key, Val, false, Ctx);
   }
 
   template <typename T, typename Context>
   void mapOptionalWithContext(const char *Key, Optional<T> &Val, Context &Ctx) {
     this->processKeyWithDefault(Key, Val, Optional<T>(), /*Required=*/false,
                                 Ctx);