Stackmaps are used for OSR exits, which is a custom kind of unwinding. Hence, they
should not be marked nounwind.

Marking them nounwind caused crashes in the WebKit FTL JIT, because if we enable
sufficient optimizations, LLVM starts eliding compact_unwind sections (or any unwind
data for that matter), making deoptimization via stackmaps impossible.

This changes the stackmap intrinsic to be may-throw, adds a test for exactly the
sympton that WebKit saw, and fixes TableGen to handle un-attributed intrinsics.

Thanks to atrick and philipreames for reviewing this.