PosixFileSystem::CopyFile: Truncate file before overwrite

[tsawada]

Without O_TRUNC, the original content of the target file will be left
over if the src file is shorter.

Fixes #28508

[gbaned]

@tsawada could you please resolve the conflicts? Thanks!

[googlebot]

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[googlebot]

CLAs look good, thanks!

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[tsawada]

Oops, I force pushed a wrong branch with ill rebasing operation. I just did another force-push that is correctly rebased on the latest master.
I can't remove reviewers as I lack such permission, but I appreciate if anyone can remove alextp, azaks2, chsigg, frankchn, jhseu and saeta from the reviewers. Sorry for the spam.

[tsawada]

Let me know if I should make another rebase to fix failing builds. It seems those builds were broken on master when I did last rebase.