pgsql: Amend recent fix for SIMILAR TO regex conversion.

Amend recent fix for SIMILAR TO regex conversion.

Commit e3ffc3e91 fixed the translation of character classes in
SIMILAR TO regular expressions.  Unfortunately the fix broke a corner
case: if there is an escape character right after the opening bracket
(for example in "[\q]"), a closing bracket right after the escape
sequence would not be seen as closing the character class.

There were two more oversights: a backslash or a nested opening bracket
right at the beginning of a character class should remove the special
meaning from any following caret or closing bracket.

This bug suggests that this code needs to be more readable, so also
rename the variables "charclass_depth" and "charclass_start" to
something more meaningful, rewrite an "if" cascade to be more
consistent, and improve the commentary.

Reported-by: Dominique Devienne <ddevienne@gmail.com>
Reported-by: Stephan Springl <springl-psql@bfw-online.de>
Author: Laurenz Albe <laurenz.albe@cybertec.at>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CAFCRh-8NwJd0jq6P=R3qhHyqU7hw0BTor3W0SvUcii24et+zAw@mail.gmail.com
Backpatch-through: 13

Branch
------
REL_15_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/9fd531534b646a62e0a99be4ca81c2f735a83293

Modified Files
--------------
src/backend/utils/adt/regexp.c        | 97 +++++++++++++++++++++++++----------
src/test/regress/expected/strings.out |  9 ++++
src/test/regress/sql/strings.sql      |  3 ++
3 files changed, 82 insertions(+), 27 deletions(-)