pgsql: Prepare tab-complete.c for preprocessing.

Prepare tab-complete.c for preprocessing.

Separate out psql_completion's giant else-if chain of *Matches
tests into a new function.  Add the infrastructure needed for
table-driven checking of the initial match of each completion
rule.  As-is, however, the code continues to operate as it did.
The new behavior applies only if SWITCH_CONVERSION_APPLIED
is #defined, which it is not here.  (The preprocessor added
in the next patch will add a #define for that.)

The first and last couple of bits of psql_completion are not
based on HeadMatches/TailMatches/Matches tests, so they stay
where they are; they won't become part of the switch.

This patch also fixes up a couple of if-conditions that didn't meet
the conditions enumerated in the comment for match_previous_words().
Those restrictions exist to simplify the preprocessor.

Discussion: https://postgr.es/m/2208466.1720729502@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/bd1276a3c9433a9e2760db6ae6e73560d7d32a22

Modified Files
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src/bin/psql/tab-complete.c      | 344 ++++++++++++++++++++++++++++++---------
src/tools/pgindent/typedefs.list |   1 +
2 files changed, 269 insertions(+), 76 deletions(-)