[asan/win] Fix incremental linking vs. global registration

The MSVC incremental linker pads every global out to 256 bytes in case
it changes size after an incremental link. So, skip over null entries in
the DSO-wide asan globals array. This only works if the global padding
size is divisible by the size of the asan global object, so add some
defensive CHECKs.

llvm-svn: 287780