[llvm] [BOLT][binary-analysis] Add initial pac-ret gadget scanner (PR #122304)

[Anatoly Trosinenko via llvm-commits]

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@@ -9,9 +9,182 @@ analyses implemented in the BOLT libraries.
 
 ## Which binary analyses are implemented?
 
-At the moment, no binary analyses are implemented.
+* [Security scanners](#security-scanners)
+  * [pac-ret analysis](#pac-ret-analysis)
 
-The goal is to make it easy using a plug-in framework to add your own analyses.
+### Security scanners
+
+For the past 25 years, a large numbers of exploits have been built and used in
+the wild to undermine computer security. The majority of these exploits abuse
+memory vulnerabilities in programs, see evidence from
+[Microsoft](https://youtu.be/PjbGojjnBZQ?si=oCHCa0SHgaSNr6Gr&t=836),
+[Chromium](https://www.chromium.org/Home/chromium-security/memory-safety/) and
+[Android](https://security.googleblog.com/2021/01/data-driven-security-hardening-in.html).
+
+It is not surprising therefore, that a large number of mitigations have been
+added to instruction sets and toolchains to make it harder to build an exploit
+using a memory vulnerability. Examples are: stack canaries, stack clash,
+pac-ret, shadow stacks, arm64e, and many more.
+
+These mitigations guarantee a so-called "security property" on the binaries they
+produce. For example, for stack canaries, the security property is roughly that
+a canary is located on the stack between the set of saved variables and the set
----------------
atrosinenko wrote:

Possibly, "between the set of saved **registers** and the set of local variables" was intended? (please feel free to ignore if I misunderstood the idea)

https://github.com/llvm/llvm-project/pull/122304