JIT breaks PostGIS

Hi,

Today I spent some time closing PostGIS tickets in preparation to Monday's
release.

One of the blockers, https://trac.osgeo.org/postgis/ticket/4125, was filed
by Postgres APT repository maintainer Christoph Berg who noticed a test
suite failure on Debian Stretch with Postgres 11.

Upon investigation we found:
- A build for Ubuntu Bionic installed on Debian Stretch passes the suite,
requiring llvm6;
- A build for Debian Stretch fails the suite on a call to external library
GEOS, showing no traces of JIT in the stacktrace;
- Setting jit=off lets the suite pass;
- The query called in clean session by itself does not crash Postgres.
Queries above it are required to reproduce the crash;
- The crash affects not only Stretch, but customly collected Postgres 11 /
clang 3.9 on Travis CI running Ubuntu Trusty:
https://github.com/postgis/postgis/pull/262.

I suspect that a fix would require to bisect llvm/clang version which stops
showing this behavior and making it a new minimum for JIT, if this is not a
symptom of bigger (memory management?) problem.

Thank you!
--
Darafei Praliaskouski
Support me: http://patreon.com/komzpa