Auto merge of #49715 - Mark-Simulacrum:deny-warnings, r=alexcrichton

Move deny(warnings) into rustbuild

This permits easier iteration without having to worry about warnings
being denied.

Fixes #49517

src/libsyntax/lib.rs

@@ -18,7 +18,6 @@
        html_favicon_url = "https://doc.rust-lang.org/favicon.ico",
        html_root_url = "https://doc.rust-lang.org/nightly/",
        test(attr(deny(warnings))))]
-#![deny(warnings)]
 
 #![feature(unicode)]
 #![feature(rustc_diagnostic_macros)]