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)

lib/SymbolGraphGen/JSON.cpp

@@ -12,6 +12,8 @@
 // Adds Symbol Graph JSON serialization to other types.
 //===----------------------------------------------------------------------===//
 
+#include "JSON.h"
+#include "Symbol.h"
 #include "swift/AST/ASTContext.h"
 #include "swift/AST/Decl.h"
 #include "swift/AST/FileUnit.h"
@@ -21,7 +23,6 @@
 #include "swift/AST/USRGeneration.h"
 #include "swift/ClangImporter/ClangModule.h"
 #include "swift/Serialization/SerializedModuleLoader.h"
-#include "JSON.h"
 
 void swift::symbolgraphgen::serialize(const llvm::VersionTuple &VT,
                                       llvm::json::OStream &OS) {
@@ -69,6 +70,8 @@ void swift::symbolgraphgen::serialize(const ExtensionDecl *Extension,
       if (const auto *ExtendedModule = ExtendedNominal->getModuleContext()) {
         OS.attribute("extendedModule", ExtendedModule->getNameStr());
       }
+
+      OS.attribute("typeKind", Symbol::getKind(ExtendedNominal).first);
     }
 
     SmallVector<Requirement, 4> FilteredRequirements;