Allow for emission of swift.extension symbols for extensions to external types in swiftSymbolGraphGen (#59047)

This includes:
 - bumping the SWIFT_SYMBOLGRAPH_FORMAT_MINOR version
 - introduction of the "swift.extension" symbol and "extensionTo" relationship
 - adding support for ExtensionDecl to the Symbol class
 - adding a "typeKind" field to the symbol's extension mixin which indicates what kind
   of symbol was extended
 - intoduction of the -emit-extension-block-symbols flag, which enables the behavior
   outlined below
 - adaptions to SymbolGraphASTWalker that ensure a swift.extension symbol is emitted
   for each extension to a type that does not exist in the local symbol graph
 - adaptions to SymbolGraph and SymbolGraphASTWalker that ensure member and conformance
   relationships are correctly associated with the swift.extension symbol instead of
   the original type declaration's (extended nominal's) symbol where applicable
 - adaptions to SymbolGraphASTWalker that ensure swift.extension symbols are connected
   to their respective extended nominal's symbol using an extensionTo relationship

Testing:
- adds SymbolGraph tests that test behavior only relevant in
  -emit-extension-block-symbols mode
- adapts some SymbolGraph tests to additionally test similar behavior for
  extensions to external types in -emit-extension-block-symbols mode
- adapts some SymbolGraph tests to (additionally or exclusively) test the
  behavior with -emit-extension-block-symbols mode enabled

Bugfixes:
- fixes a bug where some conformsTo relationships implicated by the conformances
  declared on an extension to an external type were not emitted
  (see test/SymbolGraph/Relationships/ConformsTo/Indirect.swift)

Further changes:
- documents the strategy for naming and associating children declared in extensions
  to typealiases (see test/SymbolGraph/Relationships/MemberOf/Typealias.swift,
  test/SymbolGraph/Symbols/Names.swift)

test/SymbolGraph/Module/NestedExtensions.swift

@@ -3,17 +3,20 @@
 // RUN: %target-build-swift %S/Inputs/NestedExtensions/B.swift -I %t -module-name B -emit-module -emit-module-path %t/
 // RUN: %target-build-swift %s -module-name NestedExtensions -emit-module -I %t -emit-module-path %t/
 
-// RUN: %target-swift-symbolgraph-extract -module-name A -I %t -pretty-print -output-dir %t
-// RUN: %target-swift-symbolgraph-extract -module-name B -I %t -pretty-print -output-dir %t
-// RUN: %target-swift-symbolgraph-extract -module-name NestedExtensions -I %t -pretty-print -output-dir %t
+// RUN: %target-swift-symbolgraph-extract -module-name A -I %t -pretty-print -output-dir %t -emit-extension-block-symbols
+// RUN: %target-swift-symbolgraph-extract -module-name B -I %t -pretty-print -output-dir %t -emit-extension-block-symbols
+// RUN: %target-swift-symbolgraph-extract -module-name NestedExtensions -I %t -pretty-print -output-dir %t -emit-extension-block-symbols
 
 // RUN: %FileCheck %s --input-file %t/B.symbols.json --check-prefix=MODULEB
 // RUN: %FileCheck %s --input-file %t/B@A.symbols.json --check-prefix=MODULEBATA
 
 // RUN: %FileCheck %s --input-file %t/NestedExtensions@A.symbols.json --check-prefix=NESTEDATA
+// RUN: %{python} -c 'import os.path; import sys; sys.exit(1 if os.path.exists(sys.argv[1]) else 0)' %t/NestedExtensions@B.symbols.json
 
 // RUN: %FileCheck %s --input-file %t/NestedExtensions.symbols.json --check-prefix=NESTED
 
+
+
 import A
 import B
 
@@ -35,10 +38,13 @@ extension AStruct.BStruct.CStruct where Thing: Equatable {
 
 // BStruct belongs to AStruct and so should only ever appear in B@A extension symbol graph files.
 // MODULEB-NOT: BStruct
-// MODULEBATA: "precise": "s:1A7AStructV1BE7BStructV"
+// MODULEB-NOT: "swift.extension"
+// MODULEBATA-DAG: "precise": "s:1A7AStructV1BE7BStructV"
+// MODULEBATA-DAG: "precise": "s:e:s:1A7AStructV1BE7BStructV"
 
 // CStruct belongs to BStruct, and BStruct belongs to AStruct, so should only appear in NestedExtension@A.
 // NESTED-NOT: BStruct
 // NESTED-NOT: CStruct
-// NESTEDATB-NOT: BStruct
-// NESTEDATA: "precise": "s:1A7AStructV1BE7BStructV16NestedExtensionsE7CStructV"
+// NESTED-NOT: "swift.extension"
+// NESTEDATA-DAG: "precise": "s:1A7AStructV1BE7BStructV16NestedExtensionsE7CStructV"
+// NESTEDATA-DAG: "precise": "s:e:s:1A7AStructV1BE7BStructV16NestedExtensionsE7CStructV"