[StackSafety] Pass summary into codegen

Summary:
The patch wraps ThinLTO index into immutable
pass which can be used by StackSafety analysis.

Reviewers: eugenis, pcc

Reviewed By: eugenis

Subscribers: hiraditya, steven_wu, dexonsmith, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80985