[lldb-vscode] Add an option for loading core files

Summary:
Currently loading core files on lldb-vscode is broken because there's a check in the attach workflow that asserts that the PID is valid, which of course fails for this case.
Hence, I'm adding a "coreFile" argument for the attach request, which does the work correctly.

I don't know how to test it effectively so that it runs on the buildbots and the debugger can in fact makes sense of it. Anyway, the change has been relatively simple.

Reviewers: labath, clayborg

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D78839