Change SIL ref_element_addr getFieldNo() to return a unique index.

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The fix is needed to have a unique AccessStorage object for each property. The AccessPath commits will contain test cases for this functionality.

If you're looking at the diff, only look at the first commit.

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@swift-ci test

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@swift-ci test source compatibility

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@swift-ci benchmark

[swift-ci]

Performance: -O

Regression	OLD	NEW	DELTA	RATIO
FlattenListFlatMap	2626	2913	+10.9%	0.90x (?)
FlattenListLoop	946	1031	+9.0%	0.92x (?)

Code size: -O

Performance: -Osize

Regression	OLD	NEW	DELTA	RATIO
NSStringConversion.UTF8	603	661	+9.6%	0.91x (?)
Improvement	OLD	NEW	DELTA	RATIO
FlattenListLoop	1543	1415	-8.3%	1.09x (?)

Code size: -Osize

Performance: -Onone

Improvement	OLD	NEW	DELTA	RATIO
TypeFlood	165	154	-6.7%	1.07x (?)

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: 6-Core 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

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I can't reproduce the SCK failures.

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@swift-ci test source compatibility

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SCK failures appear unrelated to this PR