[UnusedMethod] Handle reflective method references

[electrum]

Handles Class.getMethod, getDeclaredMethod, getConstructor, getDeclaredConstructor, and MethodHandles.Lookup.findStatic, findVirtual, findSpecial, findConstructor. Only suppresses findings when the method name, owner class, and parameter types can all be statically resolved.

This is cleaner than adding @Keep everywhere, and avoids the problem that methods with @Keep can later become unused. IntelliJ IDEA understands static reflection as method usage, and this gives Error Prone the same ability.

[cushon]

This approach only handles reflective references from the same file as the method declaration. Error Prone only analyzes one file at a time, it won't be able to detect reflective references from other files. So this isn't a replacement for @Keep, @Keep is still necessary for references from other files, and reflective references to methods declared in the same file are usually rarer.

[electrum]

@cushon For Trino, using references in the same file to lookup MethodHandle is very common. We have over 100 such usages, and probably zero usages of reflection across files. Here's an example file. This feature would allow us to remove ~173 usages of our @Keep-extending @UsedByGeneratedCode annotation.