Don't convert interface types (to structs) as part of CodeGenTypes.

Don't convert interface types (to structs) as part of CodeGenTypes.
 - This has pros and cons, but for now the pros seem to significantly
   outway the con.

The con is that we will always need to cast in the runtime
implementation to a struct type, if we wish to access an interface
directly.

The pros are:
 - Avoid the cost of generating types which are used. Most
   manipulation of Objective-C objects is done through messages, and
   only the implementation of a class will directly access
   memory. Previously, we would convert the type even if it only
   appear as a function parameter, for example.

 - We don't need to worry about incomplete types, and
   UpdateCompletedType for interfaces is gone.

 - It becomes easier to narrow the interface to the shadow struct for
   Objective-C interfaces (so it can be eliminated).

Currently the runtimes still use the CodeGenTypes machinery to
generate the LLVM structure they need via ConvertTagDecl, but this can
eventually be replaced.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@69797 91177308-0d34-0410-b5e6-96231b3b80d8