Have lldb-vscode update the currently selecte thread and frame when it receives a "scopes" request.

Summary: The IDE has no packets that are sent to lldb-vscode that say which thread and frame are selected. The only way we know is we get a request for variables for a stack frame via a "scopes" request. When we receive this packet we make that thread and frame the selected thread and frame in lldb. This way when people execute lldb commands in the debug console by prefixing the expression with the backtick character, we will have the right thread and frame selected. Previously this was not updated as new stack frames were selected.

Reviewers: labath, aadsm, wallace, JDevlieghere

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D77347