swift234

AST: Compress the mangling for generic signatures a bit.

Single generic parameters are common, as are a lack of requirements, so tweak things so that generic parameter counts are mangled as (count - 1), with a special mangling for zero, and give a single generic parameter at depth zero the empty-string mangling. Most requirements are protocol constraints, so use a better mangling for them that doesn't require the 'P..._' wrapping of the general type mangling. On the other hand, dependent member types ought to mangle in the protocol of the associated type, which adds some length, but isn't too bad since the protocol will almost definitely have a substitution introduced by a preceding protocol constraint.

Swift SVN r28296