[lldb-dap] Provide `declarationLocation` for variables (#102928)

This commit implements support for the "declaration location" recently
added by microsoft/debug-adapter-protocol#494 to the debug adapter
protocol.

For the `declarationLocationReference` we need a variable ID similar to
the `variablesReference`. I decided to simply reuse the
`variablesReference` here and renamed `Variables::expandable_variables`
and friends accordingly. Given that almost all variables have a
declaration location, we now assign those variable ids to all variables.

While `declarationLocationReference` effectively supersedes
`$__lldb_extensions.declaration`, I did not remove this extension, yet,
since I assume that there are some closed-source extensions which rely
on it.

I tested this against VS-Code Insiders. However, VS-Code Insiders
currently only supports `valueLoctionReference` and not
`declarationLocationReference`, yet. Locally, I hence published the
declaration locations as value locations, and VS Code Insiders navigated
to the expected places. Looking forward to proper VS Code support for
`declarationLocationReference`.

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