It decides which functions need stack protection.
It sets the `needStackProtection` flags on all function which contain stack-allocated values for which an buffer overflow could occur.
Within safe swift code there shouldn't be any buffer overflows.
But if the address of a stack variable is converted to an unsafe pointer, it's not in the control of the compiler anymore.
This means, if there is any `address_to_pointer` instruction for an `alloc_stack`, such a function is marked for stack protection.
Another case is `index_addr` for non-tail allocated memory.
This pattern appears if pointer arithmetic is done with unsafe pointers in swift code.
If the origin of an unsafe pointer can only be tracked to a function argument, the pass tries to find the root stack allocation for such an argument by doing an inter-procedural analysis.
If this is not possible, the fallback is to move the argument into a temporary `alloc_stack` and do the unsafe pointer operations on the temporary.
rdar://93677524
Adds frontend option -enable-stack-protector to enable emission of a
stack protector.
Disabled by default.
When enabled enables LLVM's strong stack protection mode.
rdar://93677524
