[WIP] Implement an intrinsic for delegate lambdas

[MichalPetryka]

Implements a basic intrinsic for creating delegate singletons, to be used by Roslyn for lambdas and method group conversions.

Creates delegates closed over null instances to save on memory, this makes it reject instance methods on generic types since those need an instance.

Uses a field for caching non frozen delegates since otherwise we'd have a noticeable perf regression on every access for cases that can't be expanded in the JIT (shared generics, unloadable assemblies). This also significantly simplifies the implementation.

TODO:

• Decide on final name and signature
• Decide if instance methods on generic types need to be supported
• Implement Mono support
• Cleanup NAOT compilation handling
• Implement support in NAOT .cctor interpreter (optional)

cc @jkotas @MichalStrehovsky @EgorBo

Depends on #99200 (without it this is a GC hole)
Depends on #125875 (fix included here too)

Closes #85014