[Serialization] Replace 'switch' in writeDecl with a DeclVisitor

[jrose-apple]

No intended functionality change; this just makes things easier to read and makes the encapsulation for helper functions a little clearer. Serializer equivalent of #22610.

[jrose-apple]

@swift-ci Please test

[jrose-apple]

I guess

@swift-ci Please smoke test compiler performance

[swift-ci]

Summary for master smoketest

Unexpected test results, excluded stats for ReactiveCocoa

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	93,461,532,937	93,476,102,885	14,569,948	0.02%
LLVM.NumLLVMBytesOutput	6,147,840	6,147,900	60	0.0%
time.swift-driver.wall	8.8s	8.7s	-96.2ms	-1.09%

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) (22)

name	old	new	delta	delta_pct
AST.NumImportedExternalDefinitions	906	906	0	0.0%
AST.NumLoadedModules	660	660	0	0.0%
AST.NumTotalClangImportedEntities	3,788	3,788	0	0.0%
IRModule.NumIRBasicBlocks	18,752	18,752	0	0.0%
IRModule.NumIRFunctions	10,522	10,522	0	0.0%
IRModule.NumIRGlobals	8,019	8,019	0	0.0%
IRModule.NumIRInsts	313,670	313,670	0	0.0%
IRModule.NumIRValueSymbols	17,602	17,602	0	0.0%
LLVM.NumLLVMBytesOutput	6,147,840	6,147,900	60	0.0%
SILModule.NumSILGenFunctions	5,370	5,370	0	0.0%
SILModule.NumSILOptFunctions	7,016	7,016	0	0.0%
Sema.NumConformancesDeserialized	13,803	13,803	0	0.0%
Sema.NumConstraintScopes	39,248	39,248	0	0.0%
Sema.NumDeclsDeserialized	117,822	117,822	0	0.0%
Sema.NumDeclsValidated	10,830	10,830	0	0.0%
Sema.NumFunctionsTypechecked	1,994	1,994	0	0.0%
Sema.NumGenericSignatureBuilders	4,075	4,075	0	0.0%
Sema.NumLazyGenericEnvironments	26,372	26,372	0	0.0%
Sema.NumLazyGenericEnvironmentsLoaded	1,986	1,986	0	0.0%
Sema.NumLazyIterableDeclContexts	18,877	18,877	0	0.0%
Sema.NumTypesDeserialized	50,085	50,085	0	0.0%
Sema.NumTypesValidated	11,642	11,642	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	141,765,989,416	141,755,785,442	-10,203,974	-0.01%
LLVM.NumLLVMBytesOutput	7,155,988	7,155,896	-92	-0.0%
time.swift-driver.wall	16.4s	16.6s	130.4ms	0.79%

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) (22)

name	old	new	delta	delta_pct
AST.NumImportedExternalDefinitions	398	398	0	0.0%
AST.NumLoadedModules	76	76	0	0.0%
AST.NumTotalClangImportedEntities	2,196	2,196	0	0.0%
IRModule.NumIRBasicBlocks	21,144	21,144	0	0.0%
IRModule.NumIRFunctions	10,004	10,004	0	0.0%
IRModule.NumIRGlobals	8,031	8,031	0	0.0%
IRModule.NumIRInsts	223,874	223,874	0	0.0%
IRModule.NumIRValueSymbols	17,207	17,207	0	0.0%
LLVM.NumLLVMBytesOutput	7,155,988	7,155,896	-92	-0.0%
SILModule.NumSILGenFunctions	4,176	4,176	0	0.0%
SILModule.NumSILOptFunctions	5,844	5,844	0	0.0%
Sema.NumConformancesDeserialized	12,333	12,333	0	0.0%
Sema.NumConstraintScopes	38,324	38,324	0	0.0%
Sema.NumDeclsDeserialized	32,593	32,593	0	0.0%
Sema.NumDeclsValidated	7,784	7,784	0	0.0%
Sema.NumFunctionsTypechecked	1,750	1,750	0	0.0%
Sema.NumGenericSignatureBuilders	1,640	1,640	0	0.0%
Sema.NumLazyGenericEnvironments	6,871	6,871	0	0.0%
Sema.NumLazyGenericEnvironmentsLoaded	130	130	0	0.0%
Sema.NumLazyIterableDeclContexts	4,283	4,283	0	0.0%
Sema.NumTypesDeserialized	17,849	17,849	0	0.0%
Sema.NumTypesValidated	6,942	6,942	0	0.0%