pgsql: Move pg_constraint.h function declarations to new file pg_constr

Move pg_constraint.h function declarations to new file pg_constraint_fn.h.

A pending patch requires exporting a function returning Bitmapset from
catalog/pg_constraint.c.  As things stand, that would mean including
nodes/bitmapset.h in pg_constraint.h, which might be hazardous for the
client-side includability of that header.  It's not entirely clear whether
any client-side code needs to include pg_constraint.h, but it seems prudent
to assume that there is some such code somewhere.  Therefore, split off the
function definitions into a new file pg_constraint_fn.h, similarly to what
we've done for some other catalog header files.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/72eee410d48dfb4e6f3a0b751c4b0057ca8adc81

Modified Files
--------------
src/backend/catalog/dependency.c       |  1 +
src/backend/catalog/heap.c             |  1 +
src/backend/catalog/index.c            |  1 +
src/backend/catalog/objectaddress.c    |  1 +
src/backend/catalog/pg_constraint.c    |  1 +
src/backend/commands/tablecmds.c       |  1 +
src/backend/commands/trigger.c         |  1 +
src/backend/commands/typecmds.c        |  1 +
src/backend/parser/parse_agg.c         |  2 +-
src/backend/parser/parse_clause.c      |  2 +-
src/backend/parser/parse_utilcmd.c     |  1 +
src/include/catalog/pg_constraint.h    | 71 +++---------------------------
src/include/catalog/pg_constraint_fn.h | 79 ++++++++++++++++++++++++++++++++++
13 files changed, 97 insertions(+), 66 deletions(-)