release-23.2: plpgsql: correctly handle parsing errors

[blathers-crl]

Backport 3/3 commits from #112200 on behalf of @DrewKimball.

/cc @cockroachdb/release

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plpgsql: correctly handle parsing errors

This patch ensures that PLpgSQL parsing errors are correctly propagated
in all cases. Previously, there were a few cases (like variable declaration
type parsing) where an error didn't halt parsing. The contract for
GetTypeFromValidSQLSyntax is also clarified, since it is ok to call with
an invalid type name as long as the error is properly handled.

Informs #105254

Release note: None

plpgsql: handle multiple expressions when one expression is expected

Previously, the PLpgSQL parser could panic when the user supplied more
than one expression in a location where only one was expected, for example,
in a return statement. This was because the PLpgSQL parser delegated to
the SQL parser's ParseExpr function, which expects exactly one input
expression. This commit returns a syntax error instead of the panic by
switching to use ParseExprs, which can handle multiple input expressions.

Informs #109342

Release note: None

plpgsql: return correct error for invalid parantheses and missing expression

This patch fixes error messages in the PLpgSQL parser for the case when
the parenthesis nesting is invalid, and for the case when no expression
(or statement) is supplied. Previously, invalid parentheses would cause
a panic without an error code, and a missing expression had the incorrect
message, since it wasn't checked until the SQL parser attempted to read
an empty string. Now, both cases are checked immediately by the PLpgSQL
parser and the correct error is propagated.

Fixes #109342

Release note: None

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Release justification: Bug fix for new features

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues or test-only changes.
• Patches should not break backwards-compatibility.
• Patches should change as little code as possible.
• Patches should not change on-disk formats or node communication protocols.
• Patches should not add new functionality.
• Patches must not add, edit, or otherwise modify cluster versions; or add version gates.

If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters.
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

Some other things to consider:

• What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
• Will this work in a cluster of mixed patch versions? Did we test that?
• If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

[cockroach-teamcity]

This change is 

[mgartner]

LGTM

[DrewKimball]

Failure is due to known flakes: #112389 and #112301