[YAMLTraits] - Revert a change committed by a mistake in D68983

I've accidentally reverted one of my previous patches.
It was not catched by bots because (I guess) they do not
build in debug (we have a test case which triggers an assert
in MSVS when runs without this change).
More info: https://reviews.llvm.org/D68983#inline-624235

Reported by Jordan Rupprecht.