pgsql: Resolve untyped parameters in FOR PORTION OF FROM/TO bounds

Resolve untyped parameters in FOR PORTION OF FROM/TO bounds

FOR PORTION OF valid_at FROM $1 TO $2 fails with "could not determine data
type of parameter $1", so using PREPARE requires users to say $1::date
etc. But transformForPortionOfClause() already coerces the bounds, since
it builds a constructor for the targeted range. We just need to store the
coerced expressions in targetFrom and targetTo.

This also improves pg_get_ruledef(), which now renders an untyped NULL
bound as NULL::date instead of NULL::unknown.

Reported-by: Noah Misch <noah(at)leadboat(dot)com>
Author: Paul A. Jungwirth <pj(at)illuminatedcomputing(dot)com>
Discussion: https://www.postgresql.org/message-id/flat/CA%2BrenyW1uJp1dw%3DiK7MsWfbbZf-dKP_4JLoQ65wj%3Dfry35FjCw%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/438f027238c0582e89d4c81e3f88d9b1d6d810d0

Modified Files
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src/backend/parser/analyze.c | 12 ++-
src/test/regress/expected/for_portion_of.out | 106 ++++++++++++++++++++++++++-
src/test/regress/sql/for_portion_of.sql | 71 ++++++++++++++++++
3 files changed, 185 insertions(+), 4 deletions(-)