[lldb] Support the Python stable C API in PythonString::AsUTF8

This conditionally reimplements PythonString::AsUTF8 using
PyUnicode_AsUTF8String instead of PyUnicode_AsUTF8AndSize.

PyUnicode_AsUTF8AndSize caches the UTF-8 representation of the string in
the Unicode object, which makes it more efficient and ties the lifetime
of the data to the Python string. However, it was only added to the
Stable API in Python 3.10. Older versions that want to use the Stable
API must use PyUnicode_AsUTF8String in combination with ConstString.

Author: JDevlieghere

lldb/source/Plugins/ScriptInterpreter/Python/PythonDataObjects.cpp

@@ -405,15 +405,33 @@ Expected<llvm::StringRef> PythonString::AsUTF8() const {
   if (!IsValid())
     return nullDeref();
 
-  Py_ssize_t size;
-  const char *data;
+  // PyUnicode_AsUTF8AndSize caches the UTF-8 representation of the string in
+  // the Unicode object, which makes it more efficient and ties the lifetime of
+  // the data to the Python string. However, it was only added to the Stable API
+  // in Python 3.10. Older versions that want to use the Stable API must use
+  // PyUnicode_AsUTF8String in combination with ConstString.
+#if defined(Py_LIMITED_API) && (Py_LIMITED_API < 0x030a0000)
+  PyObject *py_bytes = PyUnicode_AsUTF8String(m_py_obj);
+  if (!py_bytes)
+      return exception();
+  auto release_py_str =
+        llvm::make_scope_exit([py_bytes] { Py_DECREF(py_bytes); });
+  Py_ssize_t size = PyBytes_Size(py_bytes);
+  const char *str = PyBytes_AsString(py_bytes);
+
+  if (!str)
+    return exception();
 
-  data = PyUnicode_AsUTF8AndSize(m_py_obj, &size);
+  return ConstString(str, size).GetStringRef();
+#else
+  Py_ssize_t size;
+  const char *str = PyUnicode_AsUTF8AndSize(m_py_obj, &size);
 
-  if (!data)
+  if (!str)
     return exception();
 
-  return llvm::StringRef(data, size);
+  return llvm::StringRef(str, size);
+#endif
 }
 
 size_t PythonString::GetSize() const {