swift234

Fix docs and comments around optional intrinsics

docs/archive/LangRef.html is flagged as unmaintained, but I didn't see
much else about optionals in the docs so why not update it now.

`emitPreconditionOptionalHasValue` had the switch logic moved from the
standard library to lib/SILGen/SILGenConvert.cpp back in 9c7417edc2cf8f8
and there's no _preconditionOptionalHasValue intrinsic anymore. Updated
the comment in the headers to clarify.