coverage additions

I just enabled --enabled-llvm on the coverage reporting machine, which
made src/backend/jit/jit.c go from 60/71 % (line/function wise) to 78/85 % ...
and src/backend/jit/llvm from not appearing at all in the report to
78/94 %.  That's a good improvement.

If there are other obvious improvements to be had, please let me know.
(We have PG_TEST_EXTRA="ssl ldap" currently, do we have any more extra
tests now?)

-- 
Álvaro Herrera