[lldb] Track the API boundary using a thread_local variable.

The reproducers currently use a static variable to track the API
boundary. This is obviously incorrect when the SB API is used
concurrently. While I do not plan to support that use-case (right now),
I do want to avoid us crashing. As a first step, correctly track API
boundaries across multiple threads.

Before this patch SB API calls made by the embedded script interpreter
would be considered "behind the API boundary" and correctly ignored.
After this patch, we need to tell the reproducers to ignore the
scripting thread as a "private thread".

Differential revision: https://reviews.llvm.org/D92811