Add a new tool named "lldb-vscode" that implements the Visual Studio Code Debug Adaptor Protocol

This patch adds a new lldb-vscode tool that speaks the Microsoft Visual Studio Code debug adaptor protocol. It has full unit tests that test all packets.

This tool can be easily packaged up into a native extension and used with Visual Studio Code, and it can also be used by Nuclide

Differential Revision: https://reviews.llvm.org/D50365

llvm-svn: 339911