[ABI] Introduce indirect symbolic references to context descriptors.

[DougGregor]

Extending the mangling of symbolic references to also include indirect
symbolic references. This allows mangled names to refer to context
descriptors (both type and protocol) not in the current source file.

For now, only permit indirect symbolic references within the current module,
because remote mirrors (among other things) is unable to handle relocations.

[DougGregor]

@swift-ci please test

[DougGregor]

@swift-ci please test source compatibility

[DougGregor]

@swift-ci please benchmark

[DougGregor]

This picks up #19903 from @jckarter and completes the main TODOs. It still punts on the remote-mirrors bit, however.

[swift-ci]

Build comment file:

Performance: -O

TEST	OLD	NEW	DELTA	RATIO
Improvement
IterateData	1749	1618	-7.5%	1.08x (?)

Performance: -Osize

TEST	OLD	NEW	DELTA	RATIO
Improvement
IterateData	1809	1592	-12.0%	1.14x

Code size: Swift libraries

TEST	OLD	NEW	DELTA	RATIO
Improvement
libswiftNetwork.dylib	167936	163840	-2.4%	1.02x
libswiftRemoteMirror.dylib	339968	335872	-1.2%	1.01x

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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