pgsql: Move inheritance expansion code into its own file

Move inheritance expansion code into its own file

This commit moves expand_inherited_tables and underlings from
optimizer/prep/prepunionc.c to optimizer/utils/inherit.c.
Also, all of the AppendRelInfo-based expression manipulation routines
are moved to optimizer/utils/appendinfo.c.

No functional code changes. One exception is the introduction of
make_append_rel_info, but that's still just moving around code.

Also, stop including <limits.h> in prepunion.c, which no longer needs
it since 3fc6e2d7f5b6. I (Álvaro) noticed this because Amit was copying
that to inherit.c, which likewise doesn't need it.

Author: Amit Langote
Discussion: https://postgr.es/m/3be67028-a00a-502c-199a-da00eec8fb6e@lab.ntt.co.jp

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b60c39759908bb2a2dbcfc108ec19bdbdcc278e8

Modified Files
--------------
src/backend/optimizer/path/allpaths.c | 3 +-
src/backend/optimizer/path/equivclass.c | 2 +-
src/backend/optimizer/path/joinrels.c | 2 +-
src/backend/optimizer/plan/planmain.c | 2 +
src/backend/optimizer/plan/planner.c | 2 +
src/backend/optimizer/prep/prepunion.c | 1204 -------------------------------
src/backend/optimizer/util/Makefile | 5 +-
src/backend/optimizer/util/appendinfo.c | 826 +++++++++++++++++++++
src/backend/optimizer/util/inherit.c | 439 +++++++++++
src/backend/optimizer/util/pathnode.c | 1 +
src/backend/optimizer/util/relnode.c | 1 +
src/backend/partitioning/partprune.c | 1 +
src/include/optimizer/appendinfo.h | 42 ++
src/include/optimizer/inherit.h | 22 +
src/include/optimizer/prep.h | 18 -
15 files changed, 1343 insertions(+), 1227 deletions(-)