[`slow_vector_initialization`]: use the source span of vec![] macro and fix another FP
Fixes#11408
<details>
<summary>Also fixes a FP when the vec initializer comes from a macro other than `vec![]`</summary>
```rs
macro_rules! x {
() => { vec![] }
}
fn f() {
let mut v = x!();
v.resize(10, 0);
}
```
This shouldn't warn. The `x!` macro might be doing other things, so just replacing `x!()` with `vec![0; 10]` is not always an option.
</details>
I added some test cases for macro expansions, however I don't think there's a way to write a test for that specific warning that appeared in the linked issue. As far as I understand, that happens when the rust-src rustup component isn't installed (so the stdlib source is unavailable) and the span points to the `vec![]` *expansion*, instead of the `vec![]` that the user wrote.
changelog: [`slow_vector_initialization`]: use the source span of `vec![]` macro
changelog: [`slow_vector_initialization`]: only warn on `vec![]` expansions and allow other macros
415ba21c3b