Remove the synchronous GoToImplementation path for C#/VB.

Navigate to a span, not a position.

Infer better types for methods/variables with names we can understand.

Fixes #15315

Find-refs should open documents in a provisional tab.

Merge all the syntax indices into one index.

	Extract context object out so it can be used by other features.

Move to a streaming model for GoToImplementation (similar to how FindRefs works).

Remove code related to reading Identifier-Location information from our persistence service.

Fix typo in csc help text

Add feature dlls to default composition

Absolutely no code actually persisted the data, so all the reading would always fail.

Reduce unnecessary task allocations.