MS ABI: Implement proper support for setjmp

On targets which use the MSVCRT, setjmp is a macro which expands to
_setjmp or _setjmpex.

_setjmp and _setjmpex have a secret, hidden argument which is not listed
in the function prototype on X64 and WoA.  This hidden argument always
seems to be the frame pointer.

_setjmpex isn't used on X86, _setjmp is magically replaced with a call
to _setjmp3.  The second argument is zero for 'normal' setjmp/longjmp
pairs, otherwise it is a count of additional variadic arguments.  This
is used when setjmp appears inside of a try or __try.

It is not safe to use a pointer to setjmp because _setjmp, _setjmpex and
_setmp3 are not compatible with setjmp.

llvm-svn: 227426