Re: Guiding principle for dropping LLVM versions?

We could go further.  With LLVM 14 as the minimum we can just use
opaque pointers everywhere, and delete more conditional code in
master.  Tested on 14-18.

I explored using the new pass manager everywhere too.  It almost
worked, but I couldn't see how to override the inlining threshold
before LLVM 16[1], even in C++, so we couldn't fix that with a
llvmjit_wrap.cpp hack.

I like this.  How can I find out if someone would shout at me for
dropping LLVM 13?

[1] https://github.com/llvm/llvm-project/commit/4fa328074efd7eefdbb314b8f6e9f855e443ca20