Add support for SEH unwinding on Windows.

Summary:
I've tested this implementation on x86-64 to ensure that it works. All
`libc++abi` tests pass, as do all `libc++` exception-related tests. ARM
still remains to be implemented (@compnerd?).

Special thanks to KJK::Hyperion for his excellent series of articles on
how EH works on x86-64 Windows. (Seriously, check it out. It's awesome.)

I'm actually not sure if this should go in as is. I particularly don't
like that I duplicated the UnwindCursor class for this special case.

Reviewers: mstorsjo, rnk, compnerd, smeenai, javed.absar

Subscribers: mgorny, kristof.beyls, christof, chrib, cfe-commits, compnerd, llvm-commits

Differential Revision: https://reviews.llvm.org/D50564

llvm-svn: 341125