[Windows FS] Allow moving files in TempFile::keep

In r338216 / D49860 TempFile::keep was extended to allow keeping across
filesystems. The aim on Windows was to have this happen in rename_internal
using the existing system API. However, to fix an issue and preserve the
idea of "renaming" not being a move, put Windows keep-across-filesystem in
TempFile::keep.

Differential Revision: https://reviews.llvm.org/D50048

llvm-svn: 338841