[llvm-branch-commits] Add IR and codegen support for deactivation symbols. (PR #133536)

[Peter Collingbourne via llvm-branch-commits]

https://github.com/pcc created https://github.com/llvm/llvm-project/pull/133536

Deactivation symbols are a mechanism for allowing object files to disable
specific instructions in other object files at link time. The initial use
case is for pointer field protection.

For more information, see the RFC:
https://discourse.llvm.org/t/rfc-deactivation-symbols/85556

TODO:
- Add tests.