[Constant Evaluator] Add support for BinaryInteger.description which converts an integer to a string

[ravikandhadai]

This patch adds an @_semantics annotation to the BinaryInteger.description function.

[ravikandhadai]

@eeckstein this PR adds an @_semantics annotation to the BinaryInteger.description stdlib function so that it can be modeled in the constant evaluator.

[ravikandhadai]

@swift-ci Please test

[ravikandhadai]

@swift-ci Please benchmark

[swift-ci]

Performance: -O

Regression	OLD	NEW	DELTA	RATIO
ObjectiveCBridgeFromNSArrayAnyObjectForced	4860	5240	+7.8%	0.93x (?)
Improvement	OLD	NEW	DELTA	RATIO
FlattenListFlatMap	9776	9110	-6.8%	1.07x (?)
NSStringConversion.UTF8	825	771	-6.5%	1.07x (?)

Code size: -O

Performance: -Osize

Regression	OLD	NEW	DELTA	RATIO
ObjectiveCBridgeStubFromNSDateRef	3710	4110	+10.8%	0.90x (?)
NSStringConversion.UTF8	765	828	+8.2%	0.92x (?)
Improvement	OLD	NEW	DELTA	RATIO
ObjectiveCBridgeFromNSSetAnyObjectToString	79500	72000	-9.4%	1.10x (?)

Code size: -Osize

Performance: -Onone

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

[eeckstein]

lgtm