release-23.1: opt: fix BPCHAR type and CASE typing

[mgartner]

Backport 1/1 commits from #129007.

/cc @cockroachdb/release

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This commit addresses inconsistencies from Postgres' behavior. First, it
makes the BPCHAR type distinct from CHAR. The former is a
blank-padded character type with no type width, meaning that it could
have any length. The latter is a blank-padded character type with a type
width of exactly 1 - it is essentially an alias of CHAR(1).
Previously, a column of type BPCHAR behaved the same as a column of
type CHAR(1) - it enforced a length limit of 1.

Second, the typing of CASE and CASE-like expressions has been fixed.
The branches of these conditionals is no longer forced to have the same
type-width.

Fixes #127889
Fixes #108360

Release note (bug fix): A bug has been fixed that caused incorrect
evaluation of CASE, COALESCE, and IF expressions with branches
producing fixed-width string-like types, such as CHAR. In addition,
the BPCHAR type has been fixed so that it no longer incorrectly
imposes a length limit of 1.

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Release justification: Low-risk bug fix.

[blathers-crl]

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[cockroach-teamcity]

This change is 

[DrewKimball]

once the merge conflicts / extra file are resolved.

Reviewed 8 of 28 files at r1.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @kev-cao, @mgartner, and @rytaft)

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pkg/sql/logictest/testdata/logic_test/grant_table line 595 at r2 (raw file):

test           public              NULL                                    schema       public   USAGE           false
test           public              NULL                                    schema       root     ALL             true
=======

Merge conflict here.

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pkg/sql/opt/xform/testdata/external/tpcc-later-stats line 638 at r2 (raw file):

<<<<<<< HEAD
 │              │    └── CASE c_credit:37 WHEN 'BC' THEN left((((((c_id:24::STRING || c_d_id:25::STRING) || c_w_id:26::STRING) || '5') || '10') || '3860.61') || c_data:44, 500) ELSE c_data:44::STRING END
=======

Looks like the optimizer tests need to be rewritten as well.

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pkg/sql/opt/xform/testdata/external/tpcc-read-committed line 1 at r2 (raw file):

import file=tpcc_schema

This file should be removed.

[mgartner]

I think this backport would require #108387 too, which has not been backported to 23.1. I'm going to close this backport for now - I don't think there's a strong reason to backport it.