[stdlib] Set, Dictionary: force-inline native iterators

[lorentey]

This is an experimental change. While looking at generated code, I noticed some inefficiencies that would be fixed by inlining the native next().

This PR currently also inlines the top-level iterator entry points; however, that may have an adverse effect on code size, so I may need to remove that.

[lorentey]

@swift-ci smoke test

[lorentey]

@swift-ci smoke benchmark

[swift-ci]

Build comment file:

Code size: -O

TEST	OLD	NEW	DELTA	RATIO
Improvement
DictionaryKeysContains.o	18963	18179	-4.1%	1.04x

Code size: -Osize

TEST	OLD	NEW	DELTA	RATIO
Improvement
DictionaryKeysContains.o	16859	16011	-5.0%	1.05x
DictionaryBridgeToObjC.o	6917	6749	-2.4%	1.02x

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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[lorentey]

Ah, this turned out to be a code size win, but performance improvements are below the threshold.

I'll take it!