Rollup merge of rust-lang#147221 - Zalathar:incremental, r=lqd

Forbid `//@ compile-flags: -Cincremental=` in tests

Tests should not try to manually enable incremental compilation with `-Cincremental`, because that typically results in stray directories being created in the repository root.

Also, if the incremental directory is not cleared, there is a risk of interference between successive runs of the same test.

Instead, use the `//@ incremental` directive, which instructs compiletest to handle the details of passing `-Cincremental` with a fresh directory.

Author: matthiaskrgr