[6.1] Wrap various convert subtasks in signpost intervals

[d-ronnqvist]

This cherry pick contains two commits:

• Explanation: This adds no-op shims for signposting and logging, making it easier for us to add low-overhead logging or performance metrics in the code. The main reason for cherry-picking this is that it could reduce future risk if another cherry-pick does any logging. The alternative would be to cherry-pick this later or to modify later cherry-picks to remove any logging of signposting in them.
• Scope: Other cherry-picks that utilize signposting or logging will fail to build on the 6.1 branch. Higher chance of merge conflicts when cherry-picking from "main" to "release/6.1".
• Issue: rdar://141210010
• Risk: Low.
• Testing: Existing automated tests pass. Manually tested signpost output using log stream --predicate 'subsystem=="org.swift.docc"' --signpost
• Reviewer: @patshaughnessy
• Original PR: Wrap various convert subtasks in signpost intervals #1112

and:

• Explanation: This fixes a regression caused by rdar://141210010 where the DocC benchmark tools failed to build.
• Scope: DocC's benchmark tools fail to build
• Issue: rdar://141185270
• Risk: Low. Separate tool only used for benchmarking.
• Testing: Existing automated tests pass. Manually tested running the DocC benchmark tools.
• Reviewer: @franklinsch
• Original PR: Update benchmark tool to same macOS version as DocC #1124

[d-ronnqvist]

@swift-ci please test