Eliminate context descriptor cache

[airspeedswift]

Joe's recent changes to how we look up metadata should have reduced the need for this. Reverts #26579

[airspeedswift]

@swift-ci please benchmark

[airspeedswift]

@swift-ci please test

[swift-ci]

Performance: -O

Regression	OLD	NEW	DELTA	RATIO
FlattenListLoop	2634	3192	+21.2%	0.83x (?)
Calculator	133	157	+18.0%	0.85x
Chars2	3000	3400	+13.3%	0.88x

Code size: -O

Performance: -Osize

Regression	OLD	NEW	DELTA	RATIO
Chars2	3050	3550	+16.4%	0.86x
DropFirstCountableRange	15	17	+13.3%	0.88x (?)
Calculator	137	150	+9.5%	0.91x (?)
Improvement	OLD	NEW	DELTA	RATIO
ArrayLiteral2	113	98	-13.3%	1.15x (?)
Set.isStrictSubset.Int.Empty	50	45	-10.0%	1.11x (?)

Code size: -Osize

Performance: -Onone

Improvement	OLD	NEW	DELTA	RATIO
ArrayOfGenericPOD2	793	712	-10.2%	1.11x (?)
ArrayOfPOD	722	650	-10.0%	1.11x (?)

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

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 56defab

[airspeedswift]

@swift-ci please test

[DougGregor]

Definitely the right thing to do. Joe's approach is far better overall, whereas this cache can be pretty large.

[mikeash]

I'm happy to see this go. Is there a convenient link to the changes that made this unnecessary?