Downgrade linker-warnings to allow-by-default

This needs more time to bake before we turn it on. Turning it on early risks people silencing the warning indefinitely, before we have the chance to make it less noisy.

Author: jyn514

compiler/rustc_lint_defs/src/builtin.rs

@@ -4091,6 +4091,7 @@ declare_lint! {
     /// ### Example
     ///
     /// ```rust,ignore (needs CLI args, platform-specific)
+    /// #[warn(linker_messages)]
     /// extern "C" {
     ///   fn foo();
     /// }
@@ -4104,17 +4105,20 @@ declare_lint! {
     ///          >>> referenced by rust_out.69edbd30df4ae57d-cgu.0
     ///          >>>               rust_out.rust_out.69edbd30df4ae57d-cgu.0.rcgu.o:(rust_out::main::h3a90094b06757803)
     ///   |
-    ///   = note: `#[warn(linker_messages)]` on by default
-    ///
+    /// note: the lint level is defined here
+    ///  --> warn.rs:1:9
+    ///   |
+    /// 1 | #![warn(linker_messages)]
+    ///   |         ^^^^^^^^^^^^^^^
     /// warning: 1 warning emitted
     /// ```
     ///
     /// ### Explanation
     ///
     /// Linkers emit platform-specific and program-specific warnings that cannot be predicted in advance by the rust compiler.
-    /// They are forwarded by default, but can be disabled by adding `#![allow(linker_messages)]` at the crate root.
+    /// They are forwarded by default, but can be disabled by adding `#![allow(linker_messages)]` at the root of a crate that requires linking.
     pub LINKER_MESSAGES,
-    Warn,
+    Allow,
     "warnings emitted at runtime by the target-specific linker program"
 }