Try letting NSDictionary duck-typecheck things instead of doing it ourselves the slow way

[Catfish-Man]

Fixes https://bugs.swift.org/browse/SR-11154

Fixes rdar://problem/53229663

[Catfish-Man]

@swift-ci please test

[Catfish-Man]

@swift-ci please benchmark

[swift-ci]

Performance: -O

Improvement	OLD	NEW	DELTA	RATIO
JSONPerfEncode	282	256	-9.2%	1.10x (?)
PlistPerfEncode	267	244	-8.6%	1.09x (?)

Code size: -O

Performance: -Osize

Regression	OLD	NEW	DELTA	RATIO
FlattenListLoop	2151	2658	+23.6%	0.81x (?)
DataSetCountSmall	74	80	+8.1%	0.93x (?)
Improvement	OLD	NEW	DELTA	RATIO
PlistPerfEncode	269	242	-10.0%	1.11x (?)
JSONPerfEncode	284	256	-9.9%	1.11x (?)

Code size: -Osize

Performance: -Onone

Improvement	OLD	NEW	DELTA	RATIO
JSONPerfEncode	293	262	-10.6%	1.12x (?)
PlistPerfEncode	276	251	-9.1%	1.10x (?)

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 mini
  Model Identifier: Macmini8,1
  Processor Name: Intel Core i7
  Processor Speed: 3.2 GHz
  Number of Processors: 1
  Total Number of Cores: 6
  L2 Cache (per Core): 256 KB
  L3 Cache: 12 MB
  Memory: 64 GB

[Catfish-Man]

@swift-ci please smoke test and merge

[Catfish-Man]

@swift-ci please smoke test and merge

[shahmishal]

@swift-ci please smoke test and merge