[llvm-branch-commits] Add pointer field protection feature. (PR #133538)

[Peter Collingbourne via llvm-branch-commits]

https://github.com/pcc created https://github.com/llvm/llvm-project/pull/133538

Pointer field protection is a use-after-free vulnerability
mitigation that works by changing how data structures' pointer
fields are stored in memory. For more information, see the RFC:
https://discourse.llvm.org/t/rfc-structure-protection-a-family-of-uaf-mitigation-techniques/85555

TODO:
- Fix test failure.
- Add more tests.
- Add documentation.