[clang] [AArch64][PAC] Support ptrauth builtins and -fptrauth-intrinsics. (PR #65996)

[Kristof Beyls via cfe-commits]

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+Pointer Authentication
+======================
+
+.. contents::
+   :local:
+
+Introduction
+------------
+
+Pointer authentication is a technology which offers strong probabilistic protection against exploiting a broad class of memory bugs to take control of program execution.  When adopted consistently in a language ABI, it provides a form of relatively fine-grained control flow integrity (CFI) check that resists both return-oriented programming (ROP) and jump-oriented programming (JOP) attacks.
+
+While pointer authentication can be implemented purely in software, direct hardware support (e.g. as provided by ARMv8.3 PAuth) can dramatically lower the execution speed and code size costs.  Similarly, while pointer authentication can be implemented on any architecture, taking advantage of the (typically) excess addressing range of a target with 64-bit pointers minimizes the impact on memory performance and can allow interoperation with existing code (by disabling pointer authentication dynamically).  This document will generally attempt to present the pointer authentication feature independent of any hardware implementation or ABI.  Considerations that are implementation-specific are clearly identified throughout.
+
+Note that there are several different terms in use:
+
+- **Pointer authentication** is a target-independent language technology.
+
+- **PAuth** (sometimes referred to as **PAC**, for Pointer Authentication Codes) is an AArch64 architecture extension that provides hardware support for pointer authentication.
----------------
kbeyls wrote:

I'm wondering if this would be a good place to state that over the years, minor variants of the PAuth architecture have been added to the AArch64 architecture. The most relevant ones for compiler implementers are probably FEAT_PAuth, FEAT_FPAC and FEAT_FPACCOMBINE?

https://github.com/llvm/llvm-project/pull/65996