perf: skip comment assignment when no comments exist

[jbedard]

In a large repo this made a minor difference in memory consumption on gazelle invocations. This is minor enough it may not be worth it, or maybe only the first commit is worth the extra code.

The first commit is simpler and has the largest gain in my large repo, but doesn't cover as wide of a range of cases.

Robot summary of results:

  ┌────────────────────────────────────────────────────┬───────────────┬─────────┐
  │                       build                        │ order() alloc │ vs base │
  ├────────────────────────────────────────────────────┼───────────────┼─────────┤
  │ base (no buildtools change)                        │ 137.6 MB      │ —       │
  ├────────────────────────────────────────────────────┼───────────────┼─────────┤
  │ commit 1 (guard: skip when no comments)            │ 60.7 MB       │ −56%    │
  ├────────────────────────────────────────────────────┼───────────────┼─────────┤
  │ commit 1 + 2 (guard + finer-grained split)         │ 42.1 MB       │ −69%    │
  └────────────────────────────────────────────────────┴───────────────┴─────────┘

Buildtools PR checklist

• The code in this PR is covered by unit/integration tests.
• I have tested these changes and provide testing instructions below.
• I have either responded to, or resolved all Gemini comments on the PR.
• I have read Google Eng Practices on Small Changes, this PR either follows these guidelines or the description provides reasoning for why they can not be followed.

Description

assignComments previously walked the full syntax tree on every parse, building preorder and postorder node lists used to attach line and suffix comments.

This change skips the walking and pre/post construction when no comments exists.

(optional) These changes were tested using the following steps

bazel tests, patching the go.mod when invoking gazelle in a large repo while profiling memory/GC

[gemini-code-assist]

Code Review

This pull request optimizes the comment assignment process in build/lex.go by skipping the AST traversal in assignComments if there are no line or suffix comments. It also conditionally populates the preorder and postorder lists in order only when the respective comment types are present. No review comments were provided, and I have no feedback to offer.

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