[Swift language features] Implement generic methods

[jkurdek]

• [Swift language features] Implement generic methods without constraints #2942
• [Swift language features] Add basic support for protocol conformance to generic methods #2943
• [Swift language features] Add basic support for PATs constraints on generic methods #2944
• [Swift language features] Implement Swift Protocol Witness Table retrieval #2957

[kotlarmilos]

Moved to #3001.