Fix #3954: name the entry point of a top-level program 'Main'

[siegfriedpammer]

Fixes #3954.

Roslyn compiles top-level statements into a synthesized Program class whose entry point is called <Main>$ — a name that cannot be declared in C#. The decompiled output kept it verbatim (escaped to _003CMain_003E_0024 when exporting a project), and since C# accepts only a method called Main as an entry point, the exported executable did not compile:

CSC : error CS5001: Program does not contain a static 'Main' method suitable for an entry point

Change: CSharpDecompiler.DoDecompile(IMethod) gives that method the name Main, next to the existing explicit-interface-implementation rename. The new IsEntryPoint helper resolves the entry point from CorHeader.EntryPointTokenOrRelativeVirtualAddress via MetadataTokenHelpers.EntityHandleOrNil and requires a MethodDefinition handle — per ECMA-335 II.25.3.3 that field is a MethodDef or a File token, the latter when the entry point lives in another module of a multi-module assembly. Per the decision recorded in #829 top-level statements are not reconstructed — this is just the level of support the output needs in order to compile.

Async top-level programs need the name in a different place. They compile to <Main>$ holding the statements plus a <Main> wrapper that only awaits it. The wrapper carries the .entrypoint marker, but it is hidden from the output (MemberIsHidden via AsyncAwaitDecompiler.IsCompilerGeneratedMainMethod), so renaming strictly the method marked .entrypoint would leave every await-using program emitting <Main>$ and still not compiling. IsEntryPoint therefore also maps the hidden wrapper to the method it awaits, guarded by settings.AsyncAwait — the same condition under which the wrapper is hidden — so the two can never both end up named Main. A classic static async Task Main is unaffected: its wrapper is hidden and its own name is already speakable.

Not a regression: the same <Main>$ output comes out of release/10.1, v9.1, and the NuGet ilspycmd 8.2.0 (2023) and 7.2.1 (2022) — the case has simply never been handled since top-level statements arrived in C# 9.

Tests: Ugly/TopLevelProgram and Ugly/TopLevelProgramAsync — the input is valid C#, and the .Expected.cs records the output we intentionally do not reproduce faithfully, which is what the Ugly suite is for. They use Run rather than RunForLibrary, because top-level statements require an executable and the entry-point token has to be set. All 8 configurations are red before the change (emitting _003CMain_003E_0024) and green after; full decompiler suite 3340 tests, 0 failures.

Beyond the fixtures, dotnet new console and an await-using variant were decompiled with the rebuilt ilspycmd: both exported projects now compile and run, where the first previously failed with CS5001.

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