Do not suggest instance members in static contexts in MA0166/MA0167

[meziantou]

Follow-up to this comment on #793, which reports a false positive in MA0166.

Problem

MA0166 reports when a TimeProvider is available in scope. To find what's available, LookupAvailableSymbols calls SemanticModel.LookupSymbols, which also returns instance members that can't be used at that point. The code that filtered them out (IsInStaticContext) only recognized static methods. As a result, MA0166 reported things like available time providers: _timeProvider, and the code fix would have produced code that doesn't compile, in:

• static properties (expression-bodied or with a getter) and static event accessors
• static constructors and static operators
• static and instance field or property initializers
• : this(...) / : base(...) constructor initializers
• members of a nested type, which were offered the outer type's instance fields

Changes

• OperationExtensions: replace IsInStaticContext with a GetStaticContext helper that handles the contexts above.
• Instance fields and properties are now only suggested when the current type is the declaring type or derives from it. Interface default implementations can use members of the interfaces they inherit from.
• Primary constructor parameters, including records', are handled explicitly. They can be used in instance members, instance initializers, base-type arguments and other partial parts of the type. They can't be used in static members or constructor initializers (CS9105).
• MA0084 (LocalVariablesShouldNotHideSymbolsAnalyzer) used IsInStaticContext, so it now uses CanUsePrimaryConstructorParameters. Side effect: it no longer reports a local in a static property, a static field initializer or a constructor initializer as hiding a primary constructor parameter.
• docs/Rules/MA0166.md: explain why the rule exists. Overloads without a TimeProvider use TimeProvider.System, so forwarding an injected provider keeps the code testable with a fake clock. This addresses the "outdated" point in the comment.

Tests

• New UseAnOverloadThatHasTimeProviderAnalyzerTests cases for each context above, plus the primary-constructor cases (available and not available).
• New LocalVariablesShouldNotHideSymbolsAnalyzerTests cases for static contexts and instance field initializers.
• The related test classes pass on all five Roslyn versions. The full suite ran 26,959 tests with 18 failures, all JsonSourceGenerationOptionsAnalyzerTests on Roslyn 4.14 with network socket errors. That class passed on a re-run.