Rename parser token REF to REF_P to avoid a symbol conflict.

Rename parser token REF to REF_P to avoid a symbol conflict.

In the latest version of Apple's macOS SDK, 
fails to compile if "REF" is #define'd as something.
Apple may or may not agree that this is a bug, and even if
they do accept the bug report I filed, they probably won't
fix it very quickly.  In the meantime, our back branches will all
fail to compile gram.y.  v15 and HEAD currently escape the problem
thanks to the refactoring done in 98e93a1fc, but that's purely
accidental.  Moreover, since that patch removed a widely-visible
inclusion of , back-patching it seems too likely to break
third-party code.

Instead, change the token's code name to REF_P, following our usual
convention for naming parser tokens that are likely to have symbol
conflicts.  The effects of that should be localized to the grammar
and immediately surrounding files, so it seems like a safer answer.

Per project policy that we want to keep recently-out-of-support
branches buildable on modern systems, back-patch all the way to 9.2.

Discussion: https://postgr.es/m/1803927.1665938411@sss.pgh.pa.us