[EnC] Optimize EnC document analysis

Opportunities for improvement:

We cache document analyses on `EditSession` instance, which is discarded every time the debugger calls `BreakStateOrCapabilitiesChanged`.

Capabilities virtually do not change -- they can only change when debugger attaches/detaches from a runtime that has different capabilities. That's rare.

Active statements change every "step". They however impact only method body analysis of active methods. If stepping within a method body, we could only reanalyze that one method body.

Start at the EditSession document-analysis cache and the debugger's BreakStateOrCapabilitiesChanged path. Trace how capability changes and active statements invalidate analyses, then verify that unchanged capabilities reuse cached results and stepping within a method limits reanalysis to the active method body.

Written by the indexing model from the issue text.