swift234

[Type checker] Synthesize member operator '==' for Equatable enums.

Rather than synthesizing a global operator '==' for Equatable enums,
synthesize a member operator, which is more idiomatic and much
cleaner.

To make sure that these synthesized operators can actually be found,
start considering operator requirements in protocols
more generally in the type checker, so that, e.g., "myEnum == myEnum"
will type-check against Equatable.== and, on successful type-check,
will call the (newly-synthesized) witness for '=='. This both makes it
easier to make sure we find the operators in, e.g., complex multi-file
and lazy-type checking scenarios, and is a step toward the
type-checking improvements described in SE-0091.