[lldb-vscode] Fix handling of RestartRequest arguments.

According to the spec, RestartRequest has an optional "arguments" field, which
is a RestartArguments object. RestartArguments has its own optional "arguments"
field, which is a (LaunchRequestArguments | AttachRequestArguments) object. So
we need to to the "arguments" lookup twice to get to the actual launch
arguments.

Differential Revision: https://reviews.llvm.org/D150392