[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
diff --git a/llvm/docs/index.rst b/llvm/docs/index.rst
index df70de0..b60dacc 100644
--- a/llvm/docs/index.rst
+++ b/llvm/docs/index.rst
@@ -302,6 +302,7 @@
    PDB/index
    CFIVerify
    SpeculativeLoadHardening
+   StackSafetyAnalysis
 
 :doc:`WritingAnLLVMPass`
    Information on how to write LLVM transformations and analyses.
@@ -438,6 +439,10 @@
 :doc:`SpeculativeLoadHardening`
   A description of the Speculative Load Hardening mitigation for Spectre v1.
 
+:doc:`StackSafetyAnalysis`
+  This document describes the design of the stack safety analysis of local
+  variables.
+
 Development Process Documentation
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