pgsql: Simplify use of AllocSetContextCreate() wrapper macro.

Simplify use of AllocSetContextCreate() wrapper macro.

We can allow this macro to accept either abbreviated or non-abbreviated
allocation parameters by making use of __VA_ARGS__.  As noted by Andres
Freund, it's unlikely that any compiler would have __builtin_constant_p
but not __VA_ARGS__, so this gives up little or no error checking, and
it avoids a minor but annoying API break for extensions.

With this change, there is no reason for anybody to call
AllocSetContextCreateExtended directly, so in HEAD I renamed it to
AllocSetContextCreateInternal.  It's probably too late for an ABI
break like that in 11, though.

Discussion: https://postgr.es/m/20181012170355.bhxi273skjt6sag4@alap3.anarazel.de

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/3d059655be7dfcb7124c6b3ce0925448de89f669

Modified Files
--------------
src/backend/access/transam/xact.c | 10 +++++-----
src/backend/utils/mmgr/aset.c     |  6 ++++--
src/backend/utils/mmgr/mcxt.c     | 10 +++++-----
src/include/utils/memutils.h      | 13 ++++++-------
4 files changed, 20 insertions(+), 19 deletions(-)