swift234

[serialization] Use canonical conformance records for substitutions...

...instead of keeping around extra deserialized copies.

Also, fix memory management for inherited conformances already in the
ASTContext, instead of just leaking them.

In the long run, conformances under substitutions should be encoded with
some kind of CONFORMANCE_REF, because it is known that these conformances
will already exist.

Swift SVN r6557