We're implementing funclet-compatible code generation for Obj-C
exceptions when using the MSVC ABI. The idea is that the Obj-C runtime
will wrap Obj-C exceptions inside C++ exceptions, which allows for
interoperability with C++ exceptions (for Obj-C++) and zero-cost
exceptions. This is the approach taken by e.g. WinObjC, and I believe it
to be the best approach for Obj-C exceptions in the MSVC ABI.
This change implements emitting the actual funclet-compatible IR. The
generic exceptions codegen already takes care of emitting the proper
catch dispatch structures, but we need to ensure proper handling of
catch parameters and scoping (emitting the catchret). Finally blocks are
handled quite differently from Itanium; they're expected to be outlined
by the frontend, which limits some control flow possibilities but also
greatly simplifies their codegen. See r334251 for further discussion of
why frontend outlining is used.
Worked on with Saleem Abdulrasool <firstname.lastname@example.org>.