PEVerify fails compiling entrypoint in metadata-only mode

[jjonescz]

Version Used: 9881daa

Steps to Reproduce:

[Fact]
public void EmitMetadataOnly_TopLevelStatements()
{
    CompileAndVerify("""
        System.Console.WriteLine("a");
        """,
        emitOptions: EmitOptions.Default.WithEmitMetadataOnly(true))
        .VerifyDiagnostics();
}

Expected Behavior: Succeeds.

Actual Behavior: PEVerify fails with "Bad token as entry point in CLR header."

Note that using Main and TestOptions.ReleaseExe fails in the same way.

Perhaps it's expected that ref assemblies are only supported by PEVerify for class libraries, not console apps.

[AlekseyTs]

We might be failing to set the entry point in metadata only mode.

[AlekseyTs]

CC @jcouv

[jjonescz]

Working on a fix by emitting entrypoint in metadata-only EXE compilation. But it's a breaking change - previously a metadata-only EXE compilation wouldn't fail if the entrypoint was missing, now it does. Alternatively, I could make it so that it doesn't fail when the entrypoint is missing and continue with the previous behavior (emitting an invalid, PEVerify-failing assembly) in that case.