[asan] Set the unhandled exception filter slightly later during startup

VS 2015 moved the priority of their exception filter initializer from
XIY to XCAA. We now set ours to XCAB, which makes it run after both CRT
versions but before user constructors, as it should.

Fixes null_deref.cc and a variety of related tests with VS 2015. Only 4
failures remain.

llvm-svn: 264006