pgsql: Correct constness of system attributes in heap.c & prerequisites

Correct constness of system attributes in heap.c & prerequisites.

This allows the compiler / linker to mark affected pages as read-only.

There's a fair number of pre-requisite changes, to allow the const
properly be propagated. Most of consts were already required for
correctness anyway, just not represented on the type-level. Arguably
we could be more aggressive in using consts in related code, but..

This requires using a few of the types underlying typedefs that
removes pointers (e.g. const NameData *) as declaring the typedefed
type constant doesn't have the same meaning (it makes the variable
const, not what it points to).

Discussion: https://postgr.es/m/20181015200754.7y7zfuzsoux2c4ya@alap3.anarazel.de

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/02a30a09f9e57a29f7bda82f5f4bfc214eed3980

Modified Files
--------------
src/backend/catalog/heap.c | 22 +++++++++++-----------
src/backend/catalog/index.c | 2 +-
src/backend/executor/spi.c | 4 ++--
src/backend/optimizer/util/plancat.c | 2 +-
src/backend/parser/parse_relation.c | 8 ++++----
src/backend/parser/parse_utilcmd.c | 2 +-
src/backend/utils/adt/expandedrecord.c | 13 +++++++------
src/backend/utils/adt/name.c | 2 +-
src/include/catalog/heap.h | 4 ++--
src/include/parser/parse_relation.h | 2 +-
src/include/utils/builtins.h | 2 +-
11 files changed, 32 insertions(+), 31 deletions(-)