AccessPath: Add init_enum_data_addr to "access projections" set.

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AccessPath was treating init_enum_data_addr as an address base, which
is not ideal. It should be able to identify the underlying enum object
as the base. This issue was caught by LoadBorrowImmutabilityChecker
during SIL verification.

Instead handle init_enum_data_addr as a access projection that does
not affect the access path. I expect this SIL pattern to disappear
with SIL opaque values, but it still needs to be handled properly
after lowering addresses.

Functionality changes:

• any user of AccessPath now sees enum initialization stores as writes
  to the underlying enum object

• SILGen now generates begin/end access markers for enum
  initialization patterns. (Originally, we did not "see through"
  init_enum_data_addr because we didn't want to generate these
  markers, but that behavior was inconsistent and problematic).

Fixes rdar://70725514 fatal error encountered during compilation;
Unknown instruction: init_enum_data_addr)

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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

Improvement	OLD	NEW	DELTA	RATIO
DataAppendDataSmallToSmall	3220	2920	-9.3%	1.10x (?)
StringBuilderLong	1080	990	-8.3%	1.09x (?)

Code size: -O

Performance: -Osize

Regression	OLD	NEW	DELTA	RATIO
FlattenListLoop	931	1551	+66.6%	0.60x (?)
Improvement	OLD	NEW	DELTA	RATIO
FlattenListFlatMap	3540	2933	-17.1%	1.21x (?)

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 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

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - eadefc0

[atrick]

@swift-ci test OS X Platform