[NFC, StackSafety] LTO tests for MTE and StackSafety

Summary:
The test demonstrates the current state of the compiler and
I am going to resolve FIXME in followup patches.

Reviewers: eugenis

Reviewed By: eugenis

Subscribers: inglorion, hiraditya, steven_wu, dexonsmith, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D80039