Fix crossgen2 incorrect handling of ldftn to external managed methods

[davidwrighton]

Fix issue in crossgen2 where external managed methods which are used as part of an ldftn instruction are referred to through a delay load import instead of a precode import.

The issue was that the special handling of external methods as a seperate import type stepped around the handling of external methods via precode or delay load.

This change

• Renames LocalMethodImport to DelayLoadMethodImport
• Removes ExternalMethodImport
• Uses DelayLoadMethodImport and PrecodeMethodImport for all method importation
• Enables DelayLoadMethodImport and PrecodeMethodImport to be used to refer to methods which are external.
• Adds tests to cover these external sorts of imports to for creation of both delegates and function pointers

[davidwrighton]

@dotnet/crossgen-contrib

[trylek]

Looks great, thanks David for setting this right! I also appreciate your diligence in adding tons of new tests everywhere you hit a snag :-).

[cshung]

Perhaps we can close this one when this is done? #13395