pgsql: Add TOAST table to pg_index.

Add TOAST table to pg_index.

This change allows pg_index rows to use out-of-line storage for the
"indexprs" and "indpred" columns, which enables use-cases such as
very large index expressions.

This system catalog was previously not given a TOAST table due to a
fear of circularity issues (see commit 96cdeae07f). Testing has
not revealed any such problems, and it seems unlikely that the
entries for system indexes could ever need out-of-line storage. In
any case, it is still early in the v18 development cycle, so
committing this now will hopefully increase the chances of finding
any unexpected problems prior to release.

Bumps catversion.

Reported-by: Jonathan Katz
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/b611015f-b423-458c-aa2d-be0e655cc1b4%40postgresql.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b52c4fc3c09ec0ec9c1f9aa676f6d74304cc2f6f

Modified Files
--------------
src/include/catalog/catversion.h | 2 +-
src/include/catalog/pg_index.h | 2 ++
src/test/regress/expected/misc_sanity.out | 8 +++-----
src/test/regress/sql/misc_sanity.sql | 4 ++--
4 files changed, 8 insertions(+), 8 deletions(-)