[LLDB] Display artificial __promise and __coro_frame variables.

See the discussion in #69309.

Author: hokein

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

@@ -41,7 +41,11 @@ CPPLanguageRuntime::CPPLanguageRuntime(Process *process)
     : LanguageRuntime(process) {}
 
 bool CPPLanguageRuntime::IsAllowedRuntimeValue(ConstString name) {
-  return name == g_this;
+  // FIXME: use a list when the list grows more.
+  return name == g_this ||
+  // Artificial coroutine-related variables emitted by clang.
+         name == ConstString("__promise") ||
+         name == ConstString("__coro_frame");
 }
 
 bool CPPLanguageRuntime::GetObjectDescription(Stream &str,

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

@@ -78,8 +78,19 @@ 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, "// Break at co_yield", 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)
         )

lldb/test/API/functionalities/data-formatter/data-formatter-stl/generic/coroutine_handle/main.cpp

@@ -33,7 +33,7 @@ struct int_generator {
   ~int_generator() { hdl.destroy(); }
 };
 
-int_generator my_generator_func() { co_yield 42; }
+int_generator my_generator_func() { co_yield 42; } // Break at co_yield
 
 // This is an empty function which we call just so the debugger has
 // a place to reliably set a breakpoint on.