Fix insufficient logic when searching for the underlying allocation
This PR fixes the logic inside the `invalid_reference_casting` lint, when trying to lint on bigger memory layout casts.
More specifically when looking for the "underlying allocation" we were wrongly assuming that when we got `&mut slice[index]` that `slice[index]` was the allocation, but it's not.
Fixes https://github.com/rust-lang/rust/issues/124685
An example directive is ignore-test. In legacy compiletest style, the header
would be written as
// ignore-test
but in ui_test style, the header would be written as
//@ ignore-test
compiletest is changed to accept only //@ directives for UI tests
(currently), and will reject and report an error if it encounters any
comments // <content> that may be parsed as an legacy compiletest-style
test header. To fix this, you should migrate to the ui_test-style header
//@ <content>.
9fce3dc685