[IR] Add llvm.structured.gep instruction (#176145)
This commit adds initial support for `@llvm.structured.gep` instruction
in Clang. This intrinsic is supposed to be used as an alternative to
ptrdiff/GEP when pointers arithmetic is invalid and only structured
access is possible.
Link to the RFC:
https://discourse.llvm.org/t/rfc-adding-instructions-to-to-carry-gep-type-traversal-information/
Previous discussion around the documentation:
https://github.com/llvm/llvm-project/pull/167883
