[JSON] Add ObjectMapper::mapOptional to validate optional data.

Currently the idiom for mapping optional fields is:
  ObjectMapper O(Val, P);
  if (!O.map("required1", Out.R1) || !O.map("required2", Out.R2))
    return false;
  O.map("optional1", Out.O1); // ignore result
  return true;

If `optional1` is present but malformed, then we won't detect/report
that error. We may even leave `Out` in an incomplete state while returning true.
Instead, we'd often prefer to ignore `optional1` if it is absent, but otherwise
behave just like map().

Differential Revision: https://reviews.llvm.org/D89128