[stdlib] don't copy array contents on removeAll(keepingCapacity: true)

[oxy]

When calling Array.removeAll(keepingCapacity: true), bypass the replaceSubrange call unless the buffer is uniquely referenced. Also, add a benchmark to measure performance for the new branch.

Based on @glessard's work in #39784.
Resolves #56321 (rdar://71809690).

[phausler]

@swift-ci please test

[oxy]

@swift-ci please benchmark

[oxy]

Benchmark result summary:

• +1-5% codesize overhead on -Osize and -O (Array.removeAll is @inlinable)
• Array.removeAll(keepingCapacity: true) is now constant-time for arrays that are not uniquely referenced. (350-600us -> 0-5us with the current benchmark)

Details:

Performance (x86_64): -O

Improvement	OLD	NEW	DELTA	RATIO
Array.removeAll.keepingCapacity.Int	601.0	0.102	-100.0%	5834.96x
Array.removeAll.keepingCapacity.Object	358.0	5.727	-98.4%	62.50x

Code size: -O

Regression	OLD	NEW	DELTA	RATIO
ArrayRemoveAll.o	7464	7896	+5.8%	0.95x
MirrorTest.o	11422	11604	+1.6%	0.98x

Code size: -Osize

Regression	OLD	NEW	DELTA	RATIO
ArrayRemoveAll.o	6969	7374	+5.8%	0.95x
StringSplitting.o	34491	36318	+5.3%	0.95x
MirrorTest.o	11298	11468	+1.5%	0.99x

[oxy]

@swift-ci please test