pgsql: plpgsql: report proper line number for errors in variable initia

plpgsql: report proper line number for errors in variable initialization.

Previously, we pointed at the surrounding block's BEGIN keyword.
If there are multiple variables being initialized in a DECLARE section,
this isn't good enough: it can be quite confusing and unhelpful.
We do know where the variable's declaration started, so it just takes
a tiny bit more error-reporting infrastructure to use that.

Discussion: https://postgr.es/m/713975.1635530414@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/acb2d7d5d2301f07d5857ee252995e62ce9e7055

Modified Files
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src/pl/plpgsql/src/expected/plpgsql_domain.out   | 14 +++++-----
src/pl/plpgsql/src/expected/plpgsql_varprops.out | 22 +++++++--------
src/pl/plpgsql/src/pl_exec.c                     | 35 ++++++++++++++++++------
src/pl/plpgsql/src/plpgsql.h                     |  1 +
src/test/regress/expected/domain.out             |  4 +--
src/test/regress/expected/plpgsql.out            |  4 +--
6 files changed, 49 insertions(+), 31 deletions(-)