Small visitor performance tweaks

[wmdietl]

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[Copilot]

Pull request overview

This PR makes small performance-oriented tweaks in visitor code paths, primarily by avoiding repeated computations and expensive operations during type-checking.

Changes:

• Add a runtime-safe SWITCH_EXPRESSION kind lookup and use it in scan to avoid per-node string/version checks.
• Avoid materializing tree.toString() in the slow-typechecking warning path by using source-position span as a cheap size proxy, and add a small cache for thrown-expression upper-bound annotations.
• Minor micro-optimizations/cleanup: cache lint option lookup in NullnessNoInitVisitor, reorder loops to short-circuit earlier in InitializationVisitor, and reduce repeated getter calls in a few hot methods.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
framework/src/main/java/org/checkerframework/framework/type/AnnotatedTypeFactory.java	Adds TODO notes about varargs expansion being tied to type-argument inference.
framework/src/main/java/org/checkerframework/common/basetype/BaseTypeVisitor.java	Main performance tweaks: switch-expression kind caching, cheaper slow-typechecking size heuristic, caching throw upper-bound annotations, and some local refactorings.
checker/src/main/java/org/checkerframework/checker/nullness/NullnessNoInitVisitor.java	Caches a lint option value and avoids repeated receiver-type queries.
checker/src/main/java/org/checkerframework/checker/initialization/InitializationVisitor.java	Reorders nested loops with labeled breaks to short-circuit after the first match.

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