[lldb] Add missing case to switch and turn on -Werror=switch for TypeSystemSwift (swiftlang#10935)

Author: adrian-prantl

lldb/source/Plugins/TypeSystem/Swift/CMakeLists.txt

@@ -13,3 +13,9 @@ add_lldb_library(lldbPluginTypeSystemSwift PLUGIN
   LINK_COMPONENTS
     Support
 )
+
+# Enforce synchronization between Swift compiler updates and LLDB via PR testing.
+check_cxx_compiler_flag("-Werror=switch" CXX_SUPPORTS_ERROR_SWITCH)
+if (CXX_SUPPORTS_ERROR_SWITCH)
+  target_compile_options(lldbPluginTypeSystemSwift PRIVATE -Werror=switch)
+endif()

lldb/source/Plugins/TypeSystem/Swift/SwiftASTContext.cpp

@@ -4858,6 +4858,7 @@ static SwiftASTContext::TypeOrDecl DeclToTypeOrDecl(swift::Decl *decl) {
     case swift::DeclKind::Module:
     case swift::DeclKind::Missing:
     case swift::DeclKind::MissingMember:
+    case swift::DeclKind::Using:
       break;
 
     case swift::DeclKind::InfixOperator: