Update the names of the exception handling sjlj instrinsics to

Update the names of the exception handling sjlj instrinsics to
llvm.eh.sjlj.* for better clarity as to their purpose and scope. Add
a description of llvm.eh.sjlj.setjmp to ExceptionHandling.html.
(llvm.eh.sjlj.longjmp documentation coming when that implementation is
added).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@71758 91177308-0d34-0410-b5e6-96231b3b80d8