Save off another closure in emitResolver

[jakebailey]

This accounts for basically all of the allocs in declaration emit (and therefore, declaration diags in the editor, where this is 30% of all allocs). For tsc -p ./src/compiler, the number of allocs is reduced from 4431617 to 3552713.

[Copilot]

Pull Request Overview

This PR reduces allocation overhead in declaration emit by caching method closures in the emitResolver.

• Introduces a newEmitResolver constructor to initialize closure fields once.
• Adds an aliasMarkingVisitor field and renames the original method to aliasMarkingVisitorWorker to avoid per-call allocations.
• Updates GetEmitResolver to use the new constructor and adjusts the interface assertion.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
internal/checker/emitresolver.go	Added newEmitResolver, aliasMarkingVisitor field, renamed method, updated interface assertion
internal/checker/checker.go	Updated GetEmitResolver to invoke newEmitResolver