[Rd] defer removal of ENCLOS, FRAME, HASHTAB, SET_ENCLOS?

[Kevin Ushey]

Hi,

The development version of R removes the symbols

    ENCLOS, FRAME, HASHTAB, SET_ENCLOS

as per r89522. Unfortunately, these symbols are used by current
versions of RStudio, and so these versions of RStudio are unable to
launch with the development versions of R. (By extension, they would
also fail to launch using R 4.6.0 when released.)

On the RStudio side, we can work to adapt to these changes as quickly
as possible, but our experience is that users will often upgrade R
without necessarily upgrading RStudio in tandem.

I'd like to propose keeping these symbols defined in R for now (even
if not publicly accessible via headers) just to ensure existing
versions of RStudio can continue to function as expected with the
upcoming release of R.

Thanks,
Kevin