lint: deny incoherent_fundamental_impls by default

Warn the ecosystem of the pending intent-to-disallow in #49799.

Author: hdhoang

src/librustc/lint/builtin.rs

@@ -208,7 +208,7 @@ declare_lint! {
 
 declare_lint! {
     pub INCOHERENT_FUNDAMENTAL_IMPLS,
-    Warn,
+    Deny,
     "potentially-conflicting impls were erroneously allowed"
 }
 

src/test/compile-fail/issue-43355.rs

@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![deny(incoherent_fundamental_impls)]
-
 pub trait Trait1<X> {
     type Output;
 }