[Constraint solver] Fix function builders with single-expression closures

[DougGregor]

Fix a few related issues involving the interaction with
single-expression closures:

• A single-expression closure can have a "return" in it; in such
  cases, disable the function-builder transform.
• Have the function builder constraint generator look through the
  "return" statement in a single-expression closure the same way as
  solution application does

Fixes rdar://problem/59045763, where we rejected some well-formed code
involving a single-expression closure with a "return" keyword.

[DougGregor]

@swift-ci please smoke test

[DougGregor]

@swift-ci please smoke test compiler performance

[DougGregor]

@swift-ci please test source compatibility

[swift-ci]

Summary for master smoketest

No regressions above thresholds

• Debug
  • debug brief
  • debug detailed
• Release
  • release brief
  • release detailed

Debug

debug brief

Regressed (0)

name	old	new	delta	delta_pct

Improved (0)

name	old	new	delta	delta_pct

Unchanged (delta < 1.0% or delta < 100.0ms) (3)

name	old	new	delta	delta_pct
Frontend.NumInstructionsExecuted	962,208,678,529	966,836,152,289	4,627,473,760	0.48%
LLVM.NumLLVMBytesOutput	50,752,884	50,752,524	-360	-0.0%
time.swift-driver.wall	79.1s	79.2s	190.1ms	0.24%

debug detailed

Regressed (0)

name	old	new	delta	delta_pct

Improved (0)

name	old	new	delta	delta_pct

Unchanged (delta < 1.0% or delta < 100.0ms) (18)

name	old	new	delta	delta_pct
AST.NumLoadedModules	6,089	6,089	0	0.0%
AST.NumTotalClangImportedEntities	162,106	162,227	121	0.07%
IRModule.NumIRBasicBlocks	189,026	189,026	0	0.0%
IRModule.NumIRFunctions	101,615	101,615	0	0.0%
IRModule.NumIRGlobals	110,101	110,101	0	0.0%
IRModule.NumIRInsts	2,076,125	2,076,125	0	0.0%
IRModule.NumIRValueSymbols	193,072	193,072	0	0.0%
LLVM.NumLLVMBytesOutput	50,752,884	50,752,524	-360	-0.0%
SILModule.NumSILGenFunctions	49,397	49,397	0	0.0%
SILModule.NumSILOptFunctions	67,748	67,748	0	0.0%
Sema.NumConformancesDeserialized	195,948	196,501	553	0.28%
Sema.NumConstraintScopes	982,920	982,988	68	0.01%
Sema.NumDeclsDeserialized	1,559,099	1,563,137	4,038	0.26%
Sema.NumFunctionsTypechecked	10,984	10,984	0	0.0%
Sema.NumGenericSignatureBuilders	27,610	27,631	21	0.08%
Sema.NumLazyIterableDeclContexts	191,250	191,443	193	0.1%
Sema.NumTypesDeserialized	492,243	492,827	584	0.12%
Sema.NumTypesValidated	31,173	31,173	0	0.0%

Release

release brief

Regressed (0)

name	old	new	delta	delta_pct

Improved (0)

name	old	new	delta	delta_pct

Unchanged (delta < 1.0% or delta < 100.0ms) (3)

name	old	new	delta	delta_pct
Frontend.NumInstructionsExecuted	1,270,108,321,297	1,269,978,283,351	-130,037,946	-0.01%
LLVM.NumLLVMBytesOutput	56,793,536	56,793,656	120	0.0%
time.swift-driver.wall	133.7s	133.4s	-259.1ms	-0.19%

release detailed

Regressed (0)

name	old	new	delta	delta_pct

Improved (0)

name	old	new	delta	delta_pct

Unchanged (delta < 1.0% or delta < 100.0ms) (18)

name	old	new	delta	delta_pct
AST.NumLoadedModules	552	552	0	0.0%
AST.NumTotalClangImportedEntities	33,368	33,368	0	0.0%
IRModule.NumIRBasicBlocks	169,463	169,463	0	0.0%
IRModule.NumIRFunctions	77,865	77,865	0	0.0%
IRModule.NumIRGlobals	88,575	88,575	0	0.0%
IRModule.NumIRInsts	1,454,155	1,454,155	0	0.0%
IRModule.NumIRValueSymbols	155,615	155,615	0	0.0%
LLVM.NumLLVMBytesOutput	56,793,536	56,793,656	120	0.0%
SILModule.NumSILGenFunctions	28,956	28,956	0	0.0%
SILModule.NumSILOptFunctions	24,763	24,763	0	0.0%
Sema.NumConformancesDeserialized	79,161	79,161	0	0.0%
Sema.NumConstraintScopes	968,017	968,017	0	0.0%
Sema.NumDeclsDeserialized	198,541	198,541	0	0.0%
Sema.NumFunctionsTypechecked	10,984	10,984	0	0.0%
Sema.NumGenericSignatureBuilders	5,291	5,291	0	0.0%
Sema.NumLazyIterableDeclContexts	24,818	24,818	0	0.0%
Sema.NumTypesDeserialized	107,203	107,203	0	0.0%
Sema.NumTypesValidated	18,705	18,705	0	0.0%