Add a macro for the declared typlen of type timetz.

Add a macro for the declared typlen of type timetz.

pg_type.typlen says 12 for the size of timetz, but sizeof(TimeTzADT)
will be 16 on most platforms due to alignment padding.  Using the
sizeof number is no problem for usages such as palloc'ing a result
datum, but in usages such as datumCopy we really ought to match
what pg_type says.  Add a macro TIMETZ_TYPLEN so that we have a
symbolic way to write that rather than hard-coding "12".

I cannot find any place where we've needed this so far, but an
upcoming patch requires it.

Author: Tom Lane <[email protected]>
Reviewed-by: Chao Li <[email protected]>
Discussion: https://postgr.es/m/2329959.1765047648@sss.pgh.pa.us