[clang-repl] Refactor locking of runtime PTU stack (NFC) (#84176)

The Interpreter locks PTUs that originate from implicit runtime code and
initialization to prevent users from undoing them accidentally.

The previous implementation seemed hacky, because it required the reader
to be familiar with the internal workings of the PTU stack. The concept
itself is a pragmatic solution and not very surprising. This patch
introduces a function for it and adds a comment.