[Amanieu]

This PR fixes interactions between Rust panics and foreign (mainly C++) exceptions.

C++ exceptions (and other FFI exceptions) can now safely unwind through Rust code:

• The FFI function causing the unwind must be marked with #[unwind(allowed)]. If this is not the case then LLVM may optimize landing pads away with the assumption that they are unreachable.
• Drop code will be executed as the exception unwinds through the stack, as with a Rust panic.
• catch_unwind will not catch the exception, instead the exception will silently continue unwinding past it.

Rust panics can now safely unwind through C++ code:

• C++ destructors will be called as the stack unwinds.
• The Rust panic can only be caught with catch (...), after which it can be either rethrown or discarded.
• C++ cannot name the type of the Rust exception object used for unwinding, which means that it can't be caught explicitly or have its contents inspected.

Tests have been added to ensure all of the above works correctly.

Some notes about non-C++ exceptions:

• pthread_cancel and pthread_exit use unwinding on glibc. This has the same behavior as a C++ exception: destructors are run but it cannot be caught by catch_unwind.
• longjmp on Windows is implemented using unwinding. Destructors are run on MSVC, but not on MinGW. In both cases the unwind cannot be caught by catch_unwind.
• As with C++ exceptions, you need to mark the relevant FFI functions with #[unwind(allowed)], otherwise LLVM will optimize out the destructors since they seem unreachable.

I haven't updated any of the documentation, so officially unwinding through FFI is still UB. However this is a step towards making it well-defined.

Fixes #65441

cc @gnzlbg
r? @alexcrichton