Swift: Add basic support for experimental using decls

Note that we cannot write tests for these at the moment. Passing
```
-enable-experimental-feature DefaultIsolationPerFile
```
to the extractor results in:
```
error: experimental feature 'DefaultIsolationPerFile' cannot be enabled in production compile
```

swift/extractor/infra/SwiftTagTraits.h

@@ -241,7 +241,7 @@ MAP(swift::Decl, DeclTag)
   MAP(swift::MissingMemberDecl, MissingMemberDeclTag)
   MAP(swift::PatternBindingDecl, PatternBindingDeclTag)
   MAP(swift::EnumCaseDecl, EnumCaseDeclTag)
-  MAP(swift::UsingDecl, void) // TODO: Swift 6.2
+  MAP(swift::UsingDecl, UsingDeclTag)
   MAP(swift::OperatorDecl, OperatorDeclTag)
     MAP(swift::InfixOperatorDecl, InfixOperatorDeclTag)
     MAP(swift::PrefixOperatorDecl, PrefixOperatorDeclTag)

swift/extractor/translators/DeclTranslator.cpp

@@ -137,6 +137,19 @@ codeql::EnumCaseDecl DeclTranslator::translateEnumCaseDecl(const swift::EnumCase
   return entry;
 }
 
+codeql::UsingDecl DeclTranslator::translateUsingDecl(const swift::UsingDecl& decl) {
+  auto entry = createEntry(decl);
+  switch (decl.getSpecifier()) {
+    case swift::UsingSpecifier::MainActor:
+      entry.is_main_actor = true;
+      break;
+    case swift::UsingSpecifier::Nonisolated:
+      entry.is_nonisolated = true;
+      break;
+  }
+  return entry;
+}
+
 codeql::EnumElementDecl DeclTranslator::translateEnumElementDecl(
     const swift::EnumElementDecl& decl) {
   auto entry = createEntry(decl);

swift/extractor/translators/DeclTranslator.h

@@ -34,6 +34,7 @@ class DeclTranslator : public AstTranslatorBase<DeclTranslator> {
   codeql::EnumDecl translateEnumDecl(const swift::EnumDecl& decl);
   codeql::ProtocolDecl translateProtocolDecl(const swift::ProtocolDecl& decl);
   codeql::EnumCaseDecl translateEnumCaseDecl(const swift::EnumCaseDecl& decl);
+  codeql::UsingDecl translateUsingDecl(const swift::UsingDecl& decl);
   codeql::EnumElementDecl translateEnumElementDecl(const swift::EnumElementDecl& decl);
   codeql::GenericTypeParamDecl translateGenericTypeParamDecl(
       const swift::GenericTypeParamDecl& decl);