Fix https://github.com/swiftlang/swift-docc/issues/1066

[icanzilb]

Bug/issue #, if applicable: #1066

Summary

Folds sequential filter and first into a single filter(where:) to prevent the extra step.

Dependencies

n/a

Testing

The optimization only takes place when running on Linux with benchmarking enabled. I.e.:

DOCC_BENCHMARK=YES [further env variables] ./docc convert [further arguments]

There isn't a specific unit test covering the change as the behavior is unchanged.

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• [n/a] Added tests
• Ran the ./bin/test script and it succeeded
• [n/a] Updated documentation if necessary

[icanzilb]

@swift-ci Please smoke test

[d-ronnqvist]

I think this is a fine change for code readability but it doesn't have any performance impact in practice since it's only applies to the lines that takes up 0.1% of the time to compute this peak memory metric on Linux.

If we wanted to address the performance of this code we should avoid splitting all the lines (which is where ~80% of the time is spent) and find the "VmPeak" line in a way that avoids new allocations.

That said, this metric is only computed once during a full documentation build and doesn't even register in a profile so the performance of this function isn't a problem.

[icanzilb]

@swift-ci please test