[lldb-vscode] stop read loop after termination

Summary:
On Linux, when executing lldb-vscode on a remote machine, lldb-vscode doesn't die after the debug session ends. It keeps trying to read JSON input to no avail.
This diff indicates lldb-vscode to stop reading after a termination event has been processed.

Reviewers: clayborg, aadsm, kusmour

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D76314