[driver] Explicitly specify `-fbuild-session-timestamp` in seconds.

Representation of the file's last modification time depends on the file
system and isn't guaranteed to be in seconds. Cast to seconds explicitly
and tighten the test case to check the magnitude of the calculated
value, so we can catch passing milliseconds or nanoseconds.

rdar://83915615

Differential Revision: https://reviews.llvm.org/D111205