[clang] [StackProtector] Clear out stack protector slot (PR #65461)

[Bill Wendling via cfe-commits]

bwendling wrote:

>From the feedback, I think this change may be a bit premature. As I mentioned, there's already a way to zero the stack upon entry to the function.

What I *really* want to do is zero out the register that held the stack guard value, like in the GCC patch @nickdesaulniers pointed out. I'm going to close this and work on the register clearing instead. (I also messed up the branch with this change. Oh boy do I love Git!)

https://github.com/llvm/llvm-project/pull/65461