[stdlib] Restore @inline(__always) on Collection.first

[airspeedswift]

This was dropped when #8990 was merged.

[airspeedswift]

@swift-ci please smoke test and merge

[dabrahams]

but was it making a difference?

[airspeedswift]

A good question (though assume you put it your PR for a reason :) but since my reverting of it was unintended I figured I should undo that first then experiment. I'll raise a separate PR to benchmark and see if it's actually helping. Possibly .last should be given the same treatment? Though I doubt we have benchmarks that exercise it much.

[dabrahams]

You could just benchmark this change and then you'd know.

The reason for all those speculative reversions was that I discovered a flaw in my measurement methodology after I had gone and added a whole bunch of @inline(__always). There's every chance that this was not having any substantial effect, or isn't, now that Roman has updated the inliner.