[clang] Don't emit the warn_dangling_lifetime_pointer diagnostic for the assignment case. (#97408)
The `lifetime_pointer` case is handled before the assignment case. In
scenarios where we have the `gsl::Pointer` attribute, we may emit the
`-Wdangling-gsl` warning for assignment cases. This means we cannot use
`-Wno-dangling-assignment` to suppress the newly-added warning, this
patch fixes it.