Silence clang's -Wtautological-constant-compare in last_write_time.pass.cpp

Previously this broke the builders, when D38101 was committed.
Silence the warning so that it can be re-landed.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@315874 91177308-0d34-0410-b5e6-96231b3b80d8