[clang] [Clang][Sema]:Fix musttail attribute on a function with not_tail_called attribute has no warning/error (PR #134465)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> > > @AaronBallman @erichkeane Thank you for your comments. There's a question here about whether to report an error or a warning here, and you're in a divided opinion. My opinion is that it is better to use the error report here. Because these are two mutually exclusive conditions, it is semantically unreasonable to set tail calls or non-tail calls even if the program is still run after a warning is reported. I still want to hear your opinions. Regarding the PR title, PR information, test cases, I will fill in the next submission. Thank you both again.
> > 
> > 
> > I favor an error in this case because the whole point to `musttail` is to _ensure_ that a tail call happens or the code is rejected: https://clang.llvm.org/docs/AttributeReference.html#musttail
> 
> I lean towards warning, only because the behavior is 'ignored attribute', and those are a specific warning group.

CC @haberman as the original author of `musttail` in Clang, but 8344675908424ee532d4ae30e5043c5a5834e02c was the original commit for this feature and *every* misuse of `musttail` is an error there. What I recall of the design discussion is that guarantees are the entire point to using the attribute. In fact, the original commit message said "The  attribute is applied to a return statement (not a function declaration), and an error is emitted if a tail call cannot be guaranteed, for example if the function signatures of caller and callee are not compatible. Guaranteed tail calls enable a class of algorithms that would otherwise use an arbitrary amount of stack space."

https://github.com/llvm/llvm-project/pull/134465