pgsql: Clean up some edge cases in plpgsql's %TYPE parsing.

Clean up some edge cases in plpgsql's %TYPE parsing.

Support referencing a composite-type variable in %TYPE.

Remove the undocumented, untested, and pretty useless ability
to have the subject of %TYPE be an (unqualified) type name.
You get the same result by just not writing %TYPE.

Add or adjust some test cases to improve code coverage here.

Discussion: https://postgr.es/m/716852.1704402127@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/43b46aae12b220b7eca8250c6c361083f35985a0

Modified Files
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src/pl/plpgsql/src/expected/plpgsql_record.out | 46 +++++++++++++++++++
src/pl/plpgsql/src/pl_comp.c                   | 61 ++++++++++----------------
src/pl/plpgsql/src/sql/plpgsql_record.sql      | 40 +++++++++++++++++
src/test/regress/expected/plpgsql.out          | 10 ++---
src/test/regress/sql/plpgsql.sql               |  8 ++--
5 files changed, 119 insertions(+), 46 deletions(-)