Add support for async main method

[MikeFH]

Adds the feature described in #1023

I had a bit of a hard time writing the test since, until now, all tests generated DLLs and we need to generate an EXE to test that the entry point is decompiled correctly. I created a second method called RunForLibrary() to replace the old one and set the necessary flags to generate DLLs. This way we can use the Run() method to create an EXE file.

At this point, I don't see any other way to identify the generated method but by its name (see Roslyn). I put the logic to identify the method in the AsyncAwaitDecompiler since more checks could be added in the future to distinguish it.

[siegfriedpammer]

Thank you for the contribution!
Please make sure to use tabs instead of spaces in new files. Thanks!
(If you have python installed, you can use https://gist.github.com/siegfriedpammer/75700ea61609eb22714d21885e4eb084 as .git/hooks/pre-commit to prevent this in the future.)

Output from the build log:

python BuildTools\tidy.py
.\ICSharpCode.Decompiler.Tests\TestCases\Pretty\AsyncMain.cs:7: Line starting with spaces. Use tabs for indentation instead!
.\ICSharpCode.Decompiler.Tests\TestCases\Pretty\AsyncMain.cs:13: Line starting with spaces. Use tabs for indentation instead!
Tidy check: failed
Command exited with code 1

[MikeFH]

Oops, sorry. I thought I was covered with the .editorconfig and VS. I used tidy.py this time.

[siegfriedpammer]

This happens to us whenever we are adding a new file. Seems to be a bug in VS because the templates do not respect the settings in .editorconfig.

[siegfriedpammer]

You should probably check, if the method is the entrypoint as well: method.Module.Assembly?.EntryPoint == method. Ideally we should check whether the method body matches the Main(args).GetAwaiter().GetResult(); pattern, but I think this is not easily possible in a static context.

[MikeFH]

Got it, I added the check.

[siegfriedpammer]

Thank you very much for your contribution to ILSpy!

[MikeFH]

Thanks, glad I could help.