Create and use an llvm.eh.sjlj.functioncontext intrinsic.

Create and use an llvm.eh.sjlj.functioncontext intrinsic.

This intrinsic is used to pass the index of the function context to the back-end
for further processing. The back-end is in charge of filling in the rest of the
entries.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@140676 91177308-0d34-0410-b5e6-96231b3b80d8