swift234

Clean up canonicalization of GenericSignatures. NFC

GenericSignature's factory method determining whether the signature
was canonical based solely on whether the types in the parameters and
requirments were canonical. While that is currently true (for legacy
reasons), it is wrong: canonicalization also needs to canonicalize
requirements, including same-type requirements, as is currently done
in the canonical signature "for mangling". Move the "this is
canonical" dependency to the point where the canonical signature is
actually computed, so we can change the definition of canonical
signatures later.

While we're here, don't eagerly compute the canonical generic
signature in GenericSignature::getASTContext().

Swift SVN r32309