pgsql: Add const decorations

Add const decorations

in index.c and indexcmds.c and some adjacent places.  This especially
makes it easier to understand for some complicated function signatures
which are the input and the output arguments.

Discussion: https://www.postgresql.org/message-id/flat/5ed89c69-f4e6-5dab-4003-63bde7460e5e%40eisentraut.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/11af63fb48d278b86aa948a5b57f136ef03c2bb7

Modified Files
--------------
src/backend/bootstrap/bootstrap.c |  2 +-
src/backend/catalog/heap.c        |  2 +-
src/backend/catalog/index.c       | 66 +++++++++++++++++++--------------------
src/backend/catalog/namespace.c   | 10 +++---
src/backend/commands/indexcmds.c  | 58 +++++++++++++++++-----------------
src/include/bootstrap/bootstrap.h |  2 +-
src/include/catalog/heap.h        |  2 +-
src/include/catalog/index.h       | 28 ++++++++---------
src/include/catalog/namespace.h   | 10 +++---
src/include/commands/defrem.h     |  8 ++---
10 files changed, 94 insertions(+), 94 deletions(-)