[benchmark] Janitor Duty, Legacy Factor: Return of the String

[palimondo]

This PR follows-up #20861. It's a next batch of benchmark clean-up to enable robust performance measurements by adjusting workloads to run in reasonable time (< 1000 μs), minimizing the accumulated error. To maintain long-term performance tracking, it applies legacy factor where necessary.

Flavor of the day: “Hail to the String, baby!”

[palimondo]

@swift-ci please benchmark

[palimondo]

@swift-ci please smoke test

[palimondo]

@eeckstein Please review 🙏

[swift-ci]

Build comment file:

Code size: -O

TEST	OLD	NEW	DELTA	RATIO
Regression
StringBuilder.o	7316	7476	+2.2%	0.98x

Code size: -Osize

TEST	OLD	NEW	DELTA	RATIO
Regression
StringBuilder.o	7206	7398	+2.7%	0.97x
Improvement
StringMatch.o	4649	4585	-1.4%	1.01x

How to read the data

The tables contain differences in performance which are larger than 8% and differences in code size which are larger than 1%.

If you see any unexpected regressions, you should consider fixing the
regressions before you merge the PR.

Noise: Sometimes the performance results (not code size!) contain false
alarms. Unexpected regressions which are marked with '(?)' are probably noise.
If you see regressions which you cannot explain you can try to run the
benchmarks again. If regressions still show up, please consult with the
performance team (@eeckstein).

Hardware Overview

  Model Name: Mac Pro
  Model Identifier: MacPro6,1
  Processor Name: 12-Core Intel Xeon E5
  Processor Speed: 2.7 GHz
  Number of Processors: 1
  Total Number of Cores: 12
  L2 Cache (per Core): 256 KB
  L3 Cache: 30 MB
  Memory: 64 GB

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