Add emitOptional(Error|Warning|Remark) functions to simplify emission with an optional location.

In some situations a diagnostic may optionally be emitted by the presence of a location, e.g. attribute and type verification. These situations currently require extra 'if(loc) emitError(...); return failure()' wrappers that make verification clunky. These new overloads take an optional location and a list of arguments to the diagnostic, and return a LogicalResult. We take the arguments directly and return LogicalResult instead of returning InFlightDiagnostic because we cannot create a valid diagnostic with a null location. This creates an awkward situation where a user may try to treat the, potentially null, diagnostic as a valid one and encounter crashes when attaching notes/etc. Below is an example of how these methods simplify some existing usages:

Before:

  if (loc)
    emitError(*loc, "this is my diagnostic with argument: ") << 5;
  return failure();

After:

  return emitOptionalError(loc, "this is my diagnostic with argument: ", 5);

PiperOrigin-RevId: 283853599