LangRef documentation for the stackmap and patchpoint intrinsics.

These still have "experimental" status, meaning we don't guarantee
backward compatibility. However, they are already actively used by the
open source WebKit project, and have started to be adopted by other
projects.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197930 91177308-0d34-0410-b5e6-96231b3b80d8