[FS] Report errors from llvm::sys::fs::rename on Windows
[FS] Report errors from llvm::sys::fs::rename on Windows

Previously we would always report success, which is pretty bogus.

I'm too lazy to write a test where rename will portably fail on all
platforms. I'm just trying to fix breakage introduced by r234597, which
happened to tickle this.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@234611 91177308-0d34-0410-b5e6-96231b3b80d8

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