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
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master

Details
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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(-)