[clang] [clang][Sema] Deprecate `global`/`unknown` in `lifetime_capture_by` (PR #196635)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> > Given this is a keyword, maybe we should consider not deprecating that, and only deprecate things that actually conflict with identifiers.
> 
> I would also be OK with keeping `this` as an argument to the attribute. I wonder what @AaronBallman 's take is on this.

I don't have strong opinions on `lifetime_capture_by(this)` vs `lifetime_capture_by_this`. The lifetime analysis is useful in C but C has no member functions and so I'm not worried about the lack of a `this` keyword in C here.

That said, I would have expected this to be written on the member function itself given that it's supposedly a type attribute. e.g.,
```
struct S {
  void func(int *ptr) [[clang::lifetime_captures(ptr)]];
  void other_func(int *ptr);
};
```
because I would have expected diagnostics here:
```
void (S::*ptr)(int *p) = &S::func; // Error, loses important lifetime capturing information
void (S::*ptr)(int *p) [[clang::lifetime_captures(p)]] = &S::func; // Warning? Picks up lifetime capturing but that seems harmless?
```
However, it currently has no type semantics despite being written in a type position, so it probably should have been a declaration attribute instead, but that's an even more disruptive design change.

https://github.com/llvm/llvm-project/pull/196635