Fix #3858: Detect automatic events on the ILAst instead of the C# AST

[siegfriedpammer]

Automatic-event recognition moves off the syntactic accessor-body patterns in PatternStatementTransform onto structural matching of the accessor ILAst, decompiled with a fixed set of settings - the same technique RecordDecompiler uses for its method bodies. Detection becomes independent of user-visible settings by construction, and a single memoized verdict drives every event-related decision: hiding the backing field, choosing the declaration form, and translating references to the backing field. A recognized automatic event is built in field-like form directly - a custom event declaration is never constructed and collapsed after the fact, and the compiler-generated accessor bodies are never decompiled.

Five self-contained commits:

1. Extract DecompileBodyForAnalysis from RecordDecompiler - pure refactor hoisting the fixed-settings analysis pipeline so other recognizers can share it.
2. Fix AutomaticEvents doesn't work when AggressiveInlining enabled #3858: Detect automatic events on the ILAst instead of the C# AST - the new AutoEventDecompiler matches the accessor shapes structurally: the csc4/Roslyn compare-exchange loop, the pre-4.0 [MethodImpl(Synchronized)] simple-combine form (including the mcs 2.x variant that evaluates this once via IL dup), and the mcs 5.x loop. On a positive verdict, DoDecompile(IEvent) sets UseCustomEvents = false so TypeSystemAstBuilder emits the field-like declaration directly; surviving accessor and backing-field attributes are added from the type system as method:/field: sections, dropping the ones the compiler puts there.
3. Emit the backing field when a name-associated event is not automatic - MemberIsHidden decides field hiding from the name association alone; when validation rejects the event, an unreferenced backing field used to vanish from the output entirely (referenced ones were re-added through the work list).
4. Replace automatic-event backing-field references during translation - the reference rewrite moves from the identifier pass in PatternStatementTransform into ExpressionBuilder.ConvertField, keyed on the verdict. This stops references belonging to a non-automatic event from being rewritten to the event name (the reference-side half of the CS0079 defect class), keeps same-typed sibling events apart via a field-identity check (the Uses of compiler-generated events getting swapped with non-generated events of same type #3575 scenario, now pinned by a fixture), and lets the qualifier logic run against the member actually printed, dropping spurious this./type qualifiers from raise sites.
5. Remove the syntactic automatic-event patterns - the four accessor patterns, their checks, and the AST-collapse helper they used are deleted (about 300 lines). Bodyless events (abstract, extern, interface members) previously depended on the patterns' no-body clause; since C# cannot express bodyless custom accessors, DoDecompile now chooses the field-like form for them directly.

Behavior notes:

• A compiler shape the matchers do not know now degrades to explicit accessors with the backing field kept in the output - compilable, instead of the previous wrong-code failure mode.
• Raise sites lose redundant qualifiers (StaticEvent() instead of StaticAutomaticEvents.StaticEvent(), A instead of this.A); fixtures updated accordingly.

Tested with the full decompiler suite locally and all CI workflows across the compiler matrix (Roslyn 1.3.2-latest, legacy csc, mcs 2.6.4/5.23). The mcs 2.6.4 gap was in fact caught by the Windows CI matrix and is covered by the dup-alias handling and its fixtures.

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