This is a follow-up patch to #96475 to detect dangling assignments for
C++ pointer-like objects (classes annotated with the
`[[gsl::Pointer]]`). Fixes#63310.
Similar to the behavior for built-in pointer types, if a temporary owner
(`[[gsl::Owner]]`) object is assigned to a pointer-like class object,
and this temporary object is destroyed at the end of the full assignment
expression, the assignee pointer is considered dangling. In such cases,
clang will emit a warning:
```
/tmp/t.cpp:7:20: warning: object backing the pointer my_string_view will be destroyed at the end of the full-expression [-Wdangling-assignment-gsl]
7 | my_string_view = CreateString();
| ^~~~~~~~~~~~~~
1 warning generated.
```
This new warning is `-Wdangling-assignment-gsl`. It is initially
disabled, but I intend to enable it by default in clang 20.
I have initially tested this patch on our internal codebase, and it has
identified many use-after-free bugs, primarily related to `string_view`.
