[stdlib] Add custom .first to Array

[airspeedswift]

A bit brute force but it avoids double bounds checks that aren't getting optimized away.

[airspeedswift]

@swift-ci please benchmark

[swift-ci]

Performance: -O

Regression	OLD	NEW	DELTA	RATIO
ArrayAppendOptionals	1340	2130	+59.0%	0.63x (?)
Improvement	OLD	NEW	DELTA	RATIO
FlattenListFlatMap	10331	7739	-25.1%	1.33x (?)

Code size: -O

Performance: -Osize

Regression	OLD	NEW	DELTA	RATIO
ArrayAppendOptionals	1340	2070	+54.5%	0.65x (?)

Code size: -Osize

Performance: -Onone

Regression	OLD	NEW	DELTA	RATIO
ObjectiveCBridgeStubFromNSDateRef	4370	4840	+10.8%	0.90x (?)
Improvement	OLD	NEW	DELTA	RATIO
LazilyFilteredArrays2	116000	67100	-42.2%	1.73x
BucketSort	12679	9245	-27.1%	1.37x

Code size: -swiftlibs

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

[airspeedswift]

Superseded by #28486