Auto merge of #124482 - spastorino:unsafe-extern-blocks, r=oli-obk

Unsafe extern blocks

This implements RFC 3484.

Tracking issue #123743 and RFC https://github.com/rust-lang/rfcs/pull/3484

This is better reviewed commit by commit.

compiler/rustc_lint_defs/src/lib.rs

@@ -626,6 +626,9 @@ pub enum BuiltinLintDiag {
     UnexpectedCfgName((Symbol, Span), Option<(Symbol, Span)>),
     UnexpectedCfgValue((Symbol, Span), Option<(Symbol, Span)>),
     DeprecatedWhereclauseLocation(Span, Option<(Span, String)>),
+    MissingUnsafeOnExtern {
+        suggestion: Span,
+    },
     SingleUseLifetime {
         /// Span of the parameter which declares this lifetime.
         param_span: Span,