Haven't looked into it that much, but my guess is that it
was introduced in some newer version of clang than what
some people are using.  It was added for the use of the
clang-pgo build, but we ended up dropping that so it should
be ok to drop this flag.

Make Raise and Subscript exprs use capi exceptions

Add some function to old style class

And fix a bunch of resulting issues (mostly just some pretty benign
assertion errors).

Less helpful than I thought for now -- the KeyErrors are thrown
by a custom class that does `raise KeyError`, so we won't benefit
from this until we can have jitted code throw (not just receive)
capi exceptions.

Import the ctypes module.

Small bjit improvements + fixes the build

Add tip on how to debug infinite recursive bugs from tp_ slots.

It wouldn't rerun cmake if cmakelists.txt got updated.

have the llvm jit start emitting some capi-style calls

The test was passing before by using the fallback behavior, but
importing ctypes successfully makes it try to use it and fails.

- Temporarily disable code for ctypes that requires class descriptors.
- Skip the attribute lookup for __new__ for the base type class and
  directly call a special function assigned to tp_new.
- Add a #define from CPython's ./configure
- Add test.

Eventually we'll want to have a more sophisticated way of
determining when to switch to capi exceptions; they do add
noticeable overhead when no exception gets thrown.

None are enabled in this commit, but add some of the
helpers for handling exceptions if they were to be thrown.

If both a metaclass and an instance of it are defined in C extensions,
the metaclass doesn't have attrs_offset != 0 or tp_dictoffset != 0
which causes the call to PyType_Ready on the instance to fail.

Refactors and cleanup in objmodel.cpp