[stack-safety] Analysis documentation

Summary:
Basic documentation of the Stack Safety Analysis.
It will be improved during review and upstream of an implementation.

Reviewers: kcc, eugenis, vlad.tsyrklevich, glider

Reviewed By: vlad.tsyrklevich

Subscribers: arphaman, llvm-commits

Differential Revision: https://reviews.llvm.org/D53336

llvm-svn: 347612