swift234

AST: Introduce new GenericEnvironment class

A GenericEnvironment stores the mapping between GenericTypeParamTypes
and context archetypes (or eventually, concrete types, once we allow
extensions to constrain a generic parameter to a concrete type).

The goals here are two-fold:

- Eliminate the GenericTypeParamDecl::getArchetype() method, and
  always use mapTypeIntoContext() instead

- Replace SILFunction::ContextGenericParams with a GenericEnvironment

This patch adds the new data type as well as serializer and AST
verifier support. but nothing else uses it yet.

Note that GenericSignature::get() now asserts if there are no
generic parameters, instead of returning null. This requires a
few tweaks here and there.