Auto merge of #160619 - Kivooeo:enable-next-solver, r=lcnr,jdonszelmann,kobzol

Enable `-Znext-solver` on nightly by default



Implementation of https://github.com/rust-lang/compiler-team/issues/1014 cc https://github.com/rust-lang/blog.rust-lang.org/pull/1896 https://github.com/rust-lang/rust/issues/160895

This enables `-Znext-solver=globally` by default in nightly, but keeps `-Znext-solver=coherence` for most tests.