Revert [Windows] Disable TrapUnreachable for Win64, add SEH_NoReturn

Revert [Windows] Disable TrapUnreachable for Win64, add SEH_NoReturn

This reverts r370525 (git commit 0bb1630685fba255fa93def92603f064c2ffd203)
Also reverts r370543 (git commit 185ddc08eed6542781040b8499ef7ad15c8ae9f4)

The approach I took only works for functions marked `noreturn`. In
general, a call that is not known to be noreturn may be followed by
unreachable for other reasons. For example, there could be multiple call
sites to a function that throws sometimes, and at some call sites, it is
known to always throw, so it is followed by unreachable. We need to
insert an `int3` in these cases to pacify the Windows unwinder.

I think this probably deserves its own standalone, Win64-only fixup pass
that runs after block placement. Implementing that will take some time,
so let's revert to TrapUnreachable in the mean time.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@370829 91177308-0d34-0410-b5e6-96231b3b80d8