Method group conversion caching redo

[pawchen]

This is to help picking up #6642

[AlekseyTs]

@pawchen Ping me once this PR is ready for a look.

[pawchen]

@AlekseyTs Thanks, it's near. This time I dropped module scoped caching, reducing the complexities by not dealing with the module builders. I run into a few issues here though:

1. ExpressionCompiler tests are not run in CI (ExpressionEvaluator.*.UnitTests are not run in CI #58030) and they are already failing without my changes, what's worse, the failing tests involve delegates
2. Have some trouble specifying the C# version in ExpressionCompiler tests, despite I created the Compilation with preview version of C#, the C# version was set back to 10.

TODOs

• Recently anonymous delegate was introduced, not sure how to dig out the type parameters in it, need to experiment a little
• Local functions?

I'll close this PR and create another one with cleaner commit history once those issues are clear.

[pawchen]

OK, because synthesized anonymous delegates are synthesized according to the target method, despite we cannot dig the type parameters out of the delegate symbol, if the target method contains type parameters, we can still detect them there.