swift234

Give extensions generic parameters and signatures within the AST.

This adds generic parameters and generic signatures to extension
declarations. The actual generic parameters just mirror what is
available on the extended type; however, it is filled in via extension
validation, which is handled lazily.

This is a NFC step toward decoupling the archetypes of extensions from
the archetypes of the extended types <rdar://problem/16974298>.

Swift SVN r20675