translations: rename warn_ to warning

The macro warn_ was named like that because it the
keyword warn is a built-in attribute and at the time
this macro was created the word 'warning' was also
taken.

However it is no longer the case and we can rename
warn_ to warning.

compiler/rustc_macros/src/lib.rs

@@ -132,7 +132,7 @@ decl_derive!(
         diag,
         help,
         note,
-        warn_,
+        warning,
         // field attributes
         skip_arg,
         primary_span,
@@ -149,7 +149,7 @@ decl_derive!(
         diag,
         help,
         note,
-        warn_,
+        warning,
         // field attributes
         skip_arg,
         primary_span,
@@ -166,7 +166,7 @@ decl_derive!(
         label,
         help,
         note,
-        warn_,
+        warning,
         suggestion,
         suggestion_short,
         suggestion_hidden,