SILGen: Fix crash when a stored property with an initial value has a reabstractable type

[slavapestov]

A constructor can constrain generic parameters more than the
type itself, either because there is a 'where' clause on the
constructor, or because the constructor is defined inside an
extension with a 'where' clause.

In this case, when the constructor calls a stored property
initializer to implicitly initialize a stored property with
an initial value, we must apply substitutions to get the
correct result type for the call.

If the original type of the stored property lowers differently
based on the abstraction pattern, for example if it is a
function type, then emitApply() would by default re-abstract
the result to the most specific abstraction pattern possible.

However, this was wrong because we store the result value into
the stored property, and a stored property's type should
always be lowered with the most generic abstraction pattern.

In practice, this meant if you have a stored property of type
(T) -> (), and an initializer with where T == String for
example, we would call the initializer, to produce a value with
lowered type (@in_guaranteed String) -> (), then thunk it to a
(@guaranteed String) -> (), and try to store the thunk into
the stored property -- which has type (@in_guaranteed String) -> ().

This would either miscompile or trigger an assertion.

To get around this, we want to bypass the orig-to-subst
conversion performed in emitApply(). My chosen solution is
to have emitApply() emit the result into a
ConvertingInitialization set up to perform a subst-to-orig
conversion.

Now that ConvertingInitialization is smart enough to
peephole away matched pairs of orig-to-subst and subst-to-orig
conversions, this always reduces to a no-op, and the
emitApply() call produces and stores a value with the
correct abstraction pattern.

Fixes rdar://problem/67419937.

[slavapestov]

@swift-ci Please test source compatibility

[slavapestov]

@swift-ci Please smoke test

[slavapestov]

@swift-ci Please test source compatibility

[slavapestov]

@swift-ci Please test source compatibility

[slavapestov]

@swift-ci Please benchmark

[swift-ci]

Performance: -O

Regression	OLD	NEW	DELTA	RATIO
DictionaryKeysContainsNative	26	30	+15.4%	0.87x (?)

Code size: -O

Regression	OLD	NEW	DELTA	RATIO
Hash.o	22047	22815	+3.5%	0.97x
LinkedList.o	2231	2263	+1.4%	0.99x
Hanoi.o	3030	3062	+1.1%	0.99x
Improvement	OLD	NEW	DELTA	RATIO
ObserverForwarderStruct.o	2434	2402	-1.3%	1.01x

Performance: -Osize

Regression	OLD	NEW	DELTA	RATIO
FlattenListFlatMap	3918	6215	+58.6%	0.63x (?)
StringBuilderLong	1350	1570	+16.3%	0.86x (?)
StringBuilderWithLongSubstring	1950	2220	+13.8%	0.88x (?)
ProtocolDispatch2	500	564	+12.8%	0.89x
SortAdjacentIntPyramids	1580	1780	+12.7%	0.89x
ProtocolDispatch	342	371	+8.5%	0.92x
ObjectiveCBridgeFromNSSetAnyObjectToStringForced	84500	91000	+7.7%	0.93x (?)

Code size: -Osize

Improvement	OLD	NEW	DELTA	RATIO
ObserverForwarderStruct.o	2728	2645	-3.0%	1.03x
ObserverClosure.o	2481	2427	-2.2%	1.02x
ObserverPartiallyAppliedMethod.o	2547	2493	-2.1%	1.02x
OpaqueConsumingUsers.o	2258	2217	-1.8%	1.02x
ObserverUnappliedMethod.o	4998	4922	-1.5%	1.02x
Hanoi.o	2919	2875	-1.5%	1.02x

Performance: -Onone

Regression	OLD	NEW	DELTA	RATIO
StringToDataLargeUnicode	9200	10000	+8.7%	0.92x (?)
Improvement	OLD	NEW	DELTA	RATIO
SevenBoom	2507	2041	-18.6%	1.23x (?)
NSError	1066	946	-11.3%	1.13x (?)

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

[slavapestov]

@swift-ci Please smoke test

[slavapestov]

@swift-ci Please test Windows platform