Fix #633: Overrides analysis and this/base keyword navigation

[siegfriedpammer]

Fixes #633.

Two commits, covering the remaining parts of the issue:

1. "Overrides" analyzer — the dual of "Overridden By": for an override method/property/event/indexer, the analyzer panel now shows the base members it overrides. The upward interface direction already exists as the "Implements" analysis, so this adds only the base-class direction. The analyzer walks the override chain member by member via InheritanceHelper.GetBaseMember instead of collecting all signature-equal base members, so it stops at new virtual shadow boundaries, which hide any further base members from overriding. No assembly-scope scan is needed; the walk is a direct type-system lookup. Tests mirror the MemberImplementsInterfaceAnalyzerTests pattern, including a shadowing case.

2. this/base keyword navigation — as primary expressions, this now references the current type and base the base type, matching IDE go-to-definition behavior (both directions together deliberately; linking only base would make the two keywords inconsistent). The override modifier already navigated to the overridden member and constructor initializers already linked this/base to the invoked constructor; new KeywordReferenceTests pin all three behaviors.

Of the issue's four points this delivers the analysis (1) and completes keyword navigation (2, 3); the last point (clicking a declaration selects the whole override chain in the tree) overlaps #819's target-picker idea and is intentionally left out.

Verified: ILSpy.Tests 1099 total / 0 failed / 3 skipped; decompiler suite 2544 total / 0 failed / 38 skipped; XPlat solution filter builds clean.

🤖 This PR was prepared by an AI agent (Claude Code) operated by @siegfriedpammer.

[Copilot]

Pull request overview

Adds the remaining pieces of issue #633 by introducing an “Overrides” analyzer (the upward base-class direction complementing “Overridden By”/“Implements”) and by improving keyword navigation so this/base primary expressions link to their respective types, matching IDE go-to-definition behavior.

Changes:

• Added a new built-in analyzer (MemberOverridesAnalyzer) that walks the override chain upward via InheritanceHelper.GetBaseMember, stopping at new virtual shadow boundaries.
• Updated TextTokenWriter.WriteKeyword so this/base primary expressions emit type references (in addition to existing constructor-initializer and override-modifier linking).
• Added NUnit tests covering the new analyzer behavior and keyword reference behaviors.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
ILSpy.Tests/TextView/KeywordReferenceTests.cs	New tests pin this/base primary-expression type references and override modifier member reference.
ILSpy.Tests/Analyzers/Library/MemberOverridesAnalyzerTests.cs	New tests validate when “Overrides” shows and what base members it returns (including shadowing boundary).
ICSharpCode.ILSpyX/Analyzers/Builtin/MemberOverridesAnalyzer.cs	New analyzer implementation yielding overridden base-class members for override members.
ICSharpCode.Decompiler/Output/TextTokenWriter.cs	Emits references for this/base primary expressions to their resolved types.

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