[stdlib] fix an indexing inconsistency

[glessard]

This iteration ranged from buffer.startIndex until buffer.count, rather than buffer.endIndex.
The latter is more coherent with the indexing model, even though the former happens to work.

No SR.

[glessard]

@swift-ci please smoke test

[glessard]

@swift-ci please benchmark

[glessard]

@swift-ci please smoke test

[swift-ci]

Performance (x86_64): -O

Regression	OLD	NEW	DELTA	RATIO
FlattenListFlatMap	4727	6769	+43.2%	0.70x (?)
Improvement	OLD	NEW	DELTA	RATIO
ObjectiveCBridgeFromNSSetAnyObjectForced	5780	5400	-6.6%	1.07x (?)

Code size: -O

Performance (x86_64): -Osize

Regression	OLD	NEW	DELTA	RATIO
String.data.LargeUnicode	102	114	+11.8%	0.89x (?)
String.data.Medium	105	114	+8.6%	0.92x (?)

Code size: -Osize

Performance (x86_64): -Onone

Regression	OLD	NEW	DELTA	RATIO
NSError	720	802	+11.4%	0.90x (?)
ErrorHandling	3820	4140	+8.4%	0.92x (?)
StringBuilderLong	2050	2210	+7.8%	0.93x (?)
ObjectiveCBridgeStubFromNSString	1806	1942	+7.5%	0.93x (?)

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

[glessard]

@swift-ci please test windows platform

[lorentey]

What a weird way to spell a range 👍