Add [opt] suffix to optimized stack frame in lldb-vscode

To help user identify optimized code This diff adds a "[opt]" suffix to
optimized stack frames in lldb-vscode. This provides consistent experience
as command line lldb.

It also adds a new "optimized" attribute to DAP stack frame object so that
it is easy to identify from telemetry than parsing trailing "[opt]".

Differential Revision: https://reviews.llvm.org/D126013