Improve branch detection for lambda functions and async/await statements

[matteoerigozzi]

Fixes #584

In case of nested compiler generated classes, only the root one presents the CompilerGeneratedAttribute

So to detect the MoveNext() method inside an async state machine we need to check recoursively the declaring type for compiler generated attribute.

I also noticed that the operand of the instruction ldfld inside an async state machine prolog can be FieldDefinition or FieldReference.
We can see the difference looking at generated IL:

After this updates we removed some unexpected branches generates by async state machines and the report is this

Here I propose a solution to remove last 2 unexpected branches, that seems to be generated invoking a method with a delegate parameter. I added more tests to check the fairness but I'm not completely sure about it.

[MarcoRossignoli]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[MarcoRossignoli]

Looks fine!Thank's!

[nycdotnet]

Thanks so much for this fix and 2.8.1 - it really cleaned up some of our coverage stats.

[MarcoRossignoli]

@nycdotnet you can follow changelog for fix/feat here here https://github.com/tonerdo/coverlet/blob/master/Documentation/Changelog.md#release-date-2020-04-02