[BoundsSafety] Bring full support for -fbounds-safety

[rapidsna]

-fbounds-safety is a C extension to enforce bounds safety to prevent out-of-bounds (OOB) memory accesses. The extension offers bounds annotations that programmers can use to attach bounds to pointers. Using this bounds information, the compiler inserts bounds checks on every pointer dereference, ensuring that the program does not access memory outside the specified bounds. The compiler requires programmers to provide enough bounds information so that the accesses can be checked at either run time or compile time — and it rejects code if it cannot.

For more details, please refer to

RFC discussions: https://discourse.llvm.org/t/rfc-enforcing-bounds-safety-in-c-fbounds-safety/70854
The design documentation: https://clang.llvm.org/docs/BoundsSafety.html
The bounds annotations provided by -fbounds-safety could potentially be useful to implement safe interoperation between C/C++ and safe languages such as Swift. Similar to lifetime dependency annotations as pitched here.

This patch contains collaborative work from @rapidsna @apple-fcloutier @patrykstefanski @delcypher @hnrklssn and from many other collaborators from the optimization and debugger side of support.

Cherry-picking #9679

[rapidsna]

@swift-ci test

[rapidsna]

@swift-ci test llvm

[rapidsna]

@swift-ci test

[rapidsna]

@swift-ci test llvm

[rapidsna]

@swift-ci test

[rapidsna]

@swift-ci test llvm

[drodriguez]

From previous experience, "test llvm" doesn't work after merge. You might want to submit a bogus PR to retest, but https://ci.swift.org/view/Pull%20Requests/job/pr-apple-llvm-project-llvm-linux/ shows the failures happening in the Linux builds of other PRs that are testing LLVM.