[clang] Allow the no_stack_protector attribute on local variables (PR #173311)

[Akira Hatanaka via cfe-commits]

ahatanak wrote:

I agree that we want this in the NRVO case too. 

> I suspect we would want to allow this attribute in the NRVO case too, assuming that we can effectively trust/audit that writes to the given buffer are in range (maybe they are if the only one writing the buffer is compiler generated code not user written code?).

Maybe I'm misunderstanding what you mean, but users can write the buffer in this case too, I think? For example, `s.a[100] = 123;`.

https://github.com/llvm/llvm-project/pull/173311