swift234

SILGen: Fix crashes when conditionally looking up generic subscripts and properties via AnyObject.

The fix for methods to lower the dynamic method type from the substituted AST type of the expression also needed to be applied to the optional chaining, subscript, and property paths.

This also exposed a problem in the Clang importer, where imported subscript accessors would get the unbound generic context type as their Self parameter type instead of the type with the correct generic parameters. Fix this by renaming the all-too-convenient ParamDecl::createSelf factory to `createUnboundSelf`, and introduce a new `createSelf` that uses the bound generic type.

Fixes rdar://problem/26447758.