[AST] Cache the override generic signatures

[theblixguy]

Follow up to #24484.

There was a small performance regression due to creating many more GSBs when computing the override generic signature of methods. This PR adds a cache to the AST to prevent getOverrideGenericSignature from re-computing the generic signature for methods each time.

[theblixguy]

cc @slavapestov

This is a very simple implementation, but let me know your thoughts!

[theblixguy]

@slavapestov I have addressed your comments. Does it look good to you now? Thanks!

[theblixguy]

Thanks @slavapestov, could you take another look?

[slavapestov]

@swift-ci Please smoke test

[slavapestov]

@swift-ci Please test compiler performance

[theblixguy]

Hmm it seems like the caching didn’t make a huge difference: https://ci.swift.org/job/swift-PR-compiler-performance-macOS/439/artifact/comment.md/*view*/

-0.29% GSBs in debug, -0.46% in release. I’m not sure if we can improve further here?

[slavapestov]

One possible improvement would be to fast-path a few cases. If the base class signature and the derived class signature have the same generic parameters and requirements, meaning the canonical signatures are equal, you don't need to create a GSB at all -- just return the base method's signature.

[slavapestov]

Also, using getOverrideGenericSignature() in synthesizeDesignatedInitOverride() would probably help too.