tree: make SET TRANSACTION a walkable statement

[rafiss]

SET TRANSACTION is now a walkable statement. This fixes a bug where
placeholders in a SET TRANSACTION statement were not resolved correctly.

The statement must be walkable for the placeholderAnnotationVisitor to
work correctly -- it is an AST visitor that relies on walkStmt to
discover all placeholders in a statement.

cockroach/pkg/sql/sem/tree/type_check.go

Lines 3324 to 3341 in 536ff94

	func ProcessPlaceholderAnnotations(
	semaCtx *SemaContext, stmt Statement, typeHints PlaceholderTypes,
	) error {
	v := placeholderAnnotationVisitor{
	types: typeHints,
	state: make([]annotationState, len(typeHints)),
	ctx: semaCtx,
	}
	for placeholder := range typeHints {
	if typeHints[placeholder] != nil {
	v.state[placeholder] = typeFromHint
	}
	}
	walkStmt(&v, stmt)
	return v.err
	}

Epic: None
Release note (bug fix): Placeholder arguments can now be used in SET TRANSACTION statements.

[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[cockroach-teamcity]

This change is 

[chrisseto]

Change LGTM but I don't really see how this fixes the bug. Is there an issue to reference or maybe a quick example that could be added to the commit message?

[rafiss]

I think the test I added is the example I had in mind. it allows the SET TRANSACTION statement to be prepared with a placeholder, but before this change, that test would fail.

what else would you like to see?

[rafiss]

I have added a code reference to the commit message.

[chrisseto]

Perfect, thanks for the clarification!

[rafiss]

tftr!

bors r+

[craig]

Build failed:

• Bazel Essential CI (Cockroach)

[rafiss]

bors r+

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)