[test] [ctime] Ignore -Wformat-zero-length warnings

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

llvm-svn: 349294