Reapply "[lldb] Update JSONTransport to use MainLoop for reading."
(#152155)
This reverts commit cd40281685f642ad879e33f3fda8d1faa136ebf4.
This also includes some updates to try to address the platforms with
failing tests.
I updated the JSONTransport and tests to use std::function instead of
llvm:unique_function. I think the tests were failing due to the
unique_function not being moved correctly in the loop on some platforms.
45d4e84454