[LifetimeSafety] Stricter warnings for params escaping to globals

Consider the following function:

```c++
#include

struct Buffer {
std::span contents() [[clang::lifetimebound]] { }
};

std::span global;

void test() {
Buffer buffer;
auto span = buffer.contents();
global = span; // WARN
}
```

We already warn for the code above as stack-owned memory is being escaped.

On the other hand, we do not warn for the following code:

```c++
#include

struct Buffer {
std::span contents() [[clang::lifetimebound]] { }
};

std::span global;

void test(Buffer *buffer) {
auto span = buffer->contents();
global = span; // no warning
}
```

While strictly speaking we cannot be sure there is a lifetime issue in the second snippet, but this function is very error prone and passing in any buffer with a non-static lifetime is potentially problematic. In strict mode, maybe we should warn for this snippet as well?