Turn off core dumps before starting the main body of test-release.sh.

Summary:
Some of the regression tests, such as those for the various sanitizers,
use huge shadow memory maps (showing up in top as 20 TiB).  If any of
those ever crashes, your test system's disk will be filled up until
everything falls over.  Set the ulimit for core dumps to 0 to prevent
this problem.

Reviewers: hans, tstellar, rovka

Reviewed By: hans

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76191