pgsql: Mark constantly allocated dest receiver as const.

Mark constantly allocated dest receiver as const.

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

Doing so requires casting constness away, as CreateDestReceiver()
returns both constant and non-constant dest receivers. That's fine
though, as any modification of the statically allocated receivers
would already have been a bug (and would now be caught on some
platforms).

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/93ca02e00560ad6553fbd8ebabb7d5349e98ade8

Modified Files
--------------
src/backend/tcop/dest.c | 33 +++++++++++++++++++++------------
1 file changed, 21 insertions(+), 12 deletions(-)