Add benchmarks for initializing Characters from literals.

[allevato]

These benchmarks test both small characters (no more than 63 bits in UTF-8 encoding, packed in an integer) and large characters (more than 63 bits, requiring a retained StringBuffer). They will be submitted first as a baseline against which to compare #6850, which implements performance improvements for initializing characters from literals.

[allevato]

@swift-ci Please benchmark

[airspeedswift]

@swift-ci Please benchmark

[airspeedswift]

@swift-ci Please smoke test

[airspeedswift]

The benchmark job will fail (bug with CI when we add new benchmarks) but can check the log to confirm the benchmark ran, it's just the comparison with previous that fails.

[gottesmm]

@airspeedswift @shahmishal Perhaps we should add a special benchmark job that just runs the tests. I think we actually might have a preset that does that.

[jrose-apple]

I see no reason why the benchmark job shouldn't just handle newly-added benchmarks.

[airspeedswift]

I agree, the job should handle new tests and just put N/A for the speedup on the new tests.

Benchmark logs show the new tests are running OK so I think this is OK to merge.