Fix some undefined beahvior in FileMapping.

This was broken when building a 32-bit native toolchain, as
shifting a size_t right by 32 is UB when sizeof(size_t) == 8.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@318462 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed