Rename {create,emit}_warning as {create,emit}_warn.

For consistency with `warn`/`struct_warn`, and also `{create,emit}_err`,
all of which use an abbreviated form.

compiler/rustc_passes/src/check_const.rs

@@ -122,7 +122,7 @@ impl<'tcx> CheckConstVisitor<'tcx> {
             // corresponding feature gate. This encourages nightly users to use feature gates when
             // possible.
             None if tcx.sess.opts.unstable_opts.unleash_the_miri_inside_of_you => {
-                tcx.dcx().emit_warning(SkippingConstChecks { span });
+                tcx.dcx().emit_warn(SkippingConstChecks { span });
                 return;
             }