Auto merge of #31120 - alexcrichton:attribute-deny-warnings, r=brson

This commit removes the `-D warnings` flag being passed through the makefiles to
all crates to instead be a crate attribute. We want these attributes always
applied for all our standard builds, and this is more amenable to Cargo-based
builds as well.

Note that all `deny(warnings)` attributes are gated with a `cfg(stage0)`
attribute currently to match the same semantics we have today

diff --cc src/librustc/lib.rs
Simple merge

diff --cc src/librustc_data_structures/lib.rs
index 20caf7dd0cfc1c59046327038f3c46a1bccad44a,2983590af324092698b9d3c878a03b64f3c4c376..e4b13ff548a4538df0efda70e71bfd671132f2ec
--- 1/src/librustc_data_structures/lib.rs
--- 2/src/librustc_data_structures/lib.rs
+++ b/src/librustc_data_structures/lib.rs
@@@ -23,7 -23,9 +23,8 @@@
  #![doc(html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",
        html_favicon_url = "https://www.rust-lang.org/favicon.ico",
        html_root_url = "https://doc.rust-lang.org/nightly/")]
+ #![cfg_attr(not(stage0), deny(warnings))]
  
 -#![feature(hashmap_hasher)]
  #![feature(nonzero)]
  #![feature(rustc_private)]
  #![feature(staged_api)]

diff --cc src/librustc_llvm/lib.rs
Simple merge

diff --cc src/librustc_resolve/lib.rs
Simple merge

diff --cc src/libserialize/lib.rs
Simple merge

diff --cc src/libstd/lib.rs
Simple merge

diff --cc src/libstd/sys/windows/process.rs
Simple merge

diff --cc src/libsyntax/print/pprust.rs
Simple merge