[lldb-vscode] Adding support for the "disassemble" request.

Instead of creating psuedo source files for each stack frame this change adopts the new DAP “disassemble” request, allowing clients to inspect assembly instructions of files with debug info in addition to files without debug info.

[[ https://microsoft.github.io/debug-adapter-protocol/specification#Requests_Disassemble | spec ]]

See attached screenshot of the disassembly view. {F28473848}

Reviewed By: wallace

Differential Revision: https://reviews.llvm.org/D156493