Rollup merge of #143708 - epage:pretty, r=compiler-errors

fix: Include frontmatter in -Zunpretty output

In the implementation (rust-lang/rust#140035), this was left as an open question for
the tracking issue (rust-lang/rust#136889).  My assumption is that this should be
carried over.

The test was carried over from rust-lang/rust#137193 which was superseded by rust-lang/rust#140035.

Thankfully, either way, `-Zunpretty` is unstable and we can always
change it even if we stabilize frontmatter.

Author: matthiaskrgr