Add pattern recognition for calls to RuntimeHelpers.RunClassConstructor

[vitek-karas]

Linker may decide to trim static .cctor from a class if it doesn't find a need to keep it. Calling System.Runtime.CompilerServices.RuntimeHelpers.RunClassConstructor is in such case unsafe as it requires the static .cctor to be present to work correctly.

Linker should be able to analyze simple patterns calling the RunClassConstructor and include the necessary static .cctor. The minimum pattern to support should be:

RuntimeHelpers.RunClassConstructor(typeof(MyClass));

Once implement we can basically revert this change in CoreLib dotnet/runtime#31718.
This is a preferred solution to the changes suggested in #966.

[marek-safar]

Interestingly this API is used more often than I thought (2 hits in libraries, 1 in coreclr).