LLVM 16 (opaque pointers)

Hi,

Here is a draft version of the long awaited patch to support LLVM 16.
It's mostly mechanical donkeywork, but it took some time: this donkey
found it quite hard to understand the mighty getelementptr
instruction[1] and the code generation well enough to figure out all
the right types, and small mistakes took some debugging effort*. I
now finally have a patch that passes all tests.

Though it's not quite ready yet, I thought I should give this status
update to report that the main task is more or less complete, since
we're starting to get quite a few emails about it (mostly from Fedora
users) and there is an entry for it on the Open Items for 16 wiki
page. Comments/review/testing welcome.

Here are some things I think I need to do next (probably after PGCon):

1. If you use non-matching clang and LLVM versions I think we might
use "clang -no-opaque-pointers" at the wrong times (I've not looked
into that interaction yet).
2. The treatment of function types is a bit inconsistent/messy and
could be tidied up.
3. There are quite a lot of extra function calls that could perhaps be
elided (ie type variables instead of LLVMTypeInt8(), and calls to
LLVMStructGetTypeAtIndex() that are not used in LLVM < 16).
4. Could use some comments.
5. I need to test with very old versions of LLVM and Clang that we
claim to support (I have several years' worth of releases around but
nothing older than 9).
6. I need to go through the types again with a fine tooth comb, and
check the test coverage to look out for eg GEP array arithmetic with
the wrong type/size that isn't being exercised.

*For anyone working with this type of IR generation code and
questioning their sanity, I can pass on some excellent advice I got
from Andres: build LLVM yourself with assertions enabled, as they
catch some classes of silly mistake that otherwise just segfault
inscrutably on execution.

[1] https://llvm.org/docs/GetElementPtr.html