Rename graph::implementation::Graph to LinkedGraph

[Zalathar]

One of the more confusing parts of the rustc_data_structures::graph module is this mysteriously-named “Graph” type, which turns out to be an older standalone graph implementation that predates the traits used by the rest of the graph module.

This graph type is still used in a couple of places (for reporting certain lifetime errors, and by certain debugging/test-only checks of the query dependency graph), but hasn't had much attention in years.

This PR renames that old graph type from implementation::Graph to linked_graph::LinkedGraph to give it a more distinct identity (and make existing uses easier to find), and adds some notes to gently discourage any further use in new code.

No functional change.

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[Zalathar]

I did also look into removing the two existing uses of this old graph type, but doing so was sufficiently non-trivial that it doesn't turn out to be an easy win.