[LLDB] Display artificial __promise and __coro_frame variables. (#71928)

See the discussion in #69309.

Author: hokein

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

@@ -34,14 +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;
+  return name == g_this || name == g_promise || name == g_coro_frame;
 }
 
 bool CPPLanguageRuntime::GetObjectDescription(Stream &str,

lldb/test/API/functionalities/data-formatter/data-formatter-stl/generic/coroutine_handle/TestCoroutineHandle.py

@@ -78,8 +78,20 @@ def do_test(self, stdlib_type):
                 ],
             )
 
-        # Run until after the `co_yield`
         process = self.process()
+
+        # Break at a coroutine body
+        lldbutil.continue_to_source_breakpoint(
+            self,
+            process,
+            "int_generator my_generator_func",
+            lldb.SBFileSpec("main.cpp", False),
+        )
+
+        # Expect artificial variables to be displayed
+        self.expect("frame variable", substrs=["__promise", "__coro_frame"])
+
+        # Run until after the `co_yield`
         lldbutil.continue_to_source_breakpoint(
             self, process, "// Break after co_yield", lldb.SBFileSpec("main.cpp", False)
         )