Explicitly disable the -Wformat-zero-length diagnostic when running
ctime tests, since one of the test cases passes zero-length format
string to strftime(). When strftime() is appropriately decorated
with __attribute__(format, ...), this caused the test to fail because
of this warning (e.g. on NetBSD).
Differential Revision: https://reviews.llvm.org/D55661
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@349294 91177308-0d34-0410-b5e6-96231b3b80d8
