clang/GlobalModuleIndex: Don't open the same file twice. Use raw_fd_ostream(fd, ...) instead.

FIXME: PathV2::unique_file() is assumed to open the file with binary mode on win32.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173330 91177308-0d34-0410-b5e6-96231b3b80d8