jit: Add support for LLVM 12.

LLVM 12, to be released in a few months, made some breaking changes to
the Orc JIT interface. OrcV2 eventually will make it easier to support
features like concurrent JIT compilation, but this commit only allows
to compile against LLVM 12.

This commit is a bit bigger than desirable. That partially is because
the V2 interface is more granular than V1 interface, but also because
I chose to make some minor changes to < LLVM 12 code to keep the code
somewhat readable.

The LLVM 12 support will need to be backpatched. I plan to do so after
the patch stewed on the buildfarm for a few days.

Author: Andres Freund
Discussion: https://postgr.es/m/20201016011244.pmyvr3ee2gbzplq4@alap3.anarazel.de

Author: anarazel