Use static constants for builtin variables.

Author: hokein

lldb/source/Plugins/LanguageRuntime/CPlusPlus/CPPLanguageRuntime.cpp

@@ -34,17 +34,17 @@ using namespace lldb;
 using namespace lldb_private;
 
 static ConstString g_this = ConstString("this");
+// Artificial coroutine-related variables emitted by clang.
+static ConstString g_promise = ConstString("__promise");
+static ConstString g_coro_frame = ConstString("__coro_frame");
 
 char CPPLanguageRuntime::ID = 0;
 
 CPPLanguageRuntime::CPPLanguageRuntime(Process *process)
     : LanguageRuntime(process) {}
 
 bool CPPLanguageRuntime::IsAllowedRuntimeValue(ConstString name) {
-  return name == g_this ||
-         // Artificial coroutine-related variables emitted by clang.
-         name == ConstString("__promise") ||
-         name == ConstString("__coro_frame");
+  return name == g_this || name == g_promise || name == g_coro_frame;
 }
 
 bool CPPLanguageRuntime::GetObjectDescription(Stream &str,