Add regression test for comment preservation in func-to-computed-property refactoring

[nenadvulic]

Refs #3236.

#3236 reports that ConvertZeroParameterFunctionToComputedProperty drops a leading comment (e.g. a doc comment) when converting a zero-parameter function to a computed property.

I couldn't reproduce this on main: the refactoring already preserves the converted declaration's leading trivia. The binding specifier is derived from the detached funcKeyword (which keeps its leading trivia), and attributes/modifiers are carried over — so the comment survives whether it sits on the func keyword, a modifier, or an attribute.

This adds a regression test (testRefactoringFunctionToComputedPropertyPreservesLeadingComment) so the behavior stays correct, covering:

• a /// doc comment with no modifiers,
• a /// comment with a modifier (public),
• a /// comment with an attribute (@inlinable),
• a /** */ block comment.

All pass on main. Given this, #3236 looks resolved and could be closed once this lands (or if someone has a precise case that still drops the comment, the test is a good place to add it).

[ahoppen]

@swift-ci Please test

[ahoppen]

Thanks, @nenadvulic.

[nenadvulic]

@ahoppen The two CI failures look unrelated to this change 😞

• Swift Test Windows Platform (Jenkins): the build failed during Get-Dependencies, before compiling anything — the dependency server returned an HTTP 500 (Exception calling "DownloadFile" ... (500) Internal Server Error). The
  GitHub Actions Windows jobs (5.9 through nightly-main) and Run tests using swift-syntax-dev-utils (Windows) all pass.

• Test / Swift SDK for Wasm Build (nightly-main - jammy): crashes with Error: Trap: out of bounds memory access in the XCTest runtime using swift-DEVELOPMENT-SNAPSHOT-2026-05-27-a. The same job fails on other recent PRs too
  (e.g. Allow trailing commas in macro role attribute arguments #3361, which only touches CODEOWNERS), so it looks like a pre-existing nightly-main toolchain issue. The Wasm jobs on 6.2 and nightly-6.3 pass.

All the jobs that actually run the new test are green.

[ahoppen]

Yeah, the failure are unrelated. Let me trigger CI again.

@swift-ci Please test Windows