tsan: fix windows meta mapping

Currently windows fails on startup with:
CHECK failed: gotsan.cc:3077 "(((m - prev_m) / kMetaShadowSize)) == (((p - prev) / kMetaShadowCell))" (0x3ffffffeffffff7e, 0x6ffffff7e)

Make MemToMeta do the same MemToShadow does on windows: add offset instead of or'ing it.

llvm-svn: 285420