Rollup merge of #70632 - tspiteri:vec-new, r=sfackler
Rollup merge of #70632 - tspiteri:vec-new, r=sfackler

expand vec![] to Vec::new()

The current expansion of `vec![]` calls `into_vec` on a boxed slice, which results in longer IR, and even after optimization, some unwinding artifacts are still present in the IR. This PR uses `Vec::new()` for `vec![]`.

This also allows `vec![]` to be used in const expressions.

graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust