[lldb-dap] Emit declarations along with variables (#74865)

This is an extension to the protocol that emits the declaration
information along with the metadata of each variable. This can be used
by vscode extensions to implement, for example, a "goToDefinition"
action in the debug tab, or for showing the value of a variable right
next to where it's declared during a debug session.
As this is cheap, I'm not gating this information under any setting.