Don't throw feature errors when it seems like someone is trying to use the extern prelude

Sometimes that someone is the compiler speculatively resolving things to
provide better error messages or lints

Author: Manishearth

src/librustc_resolve/lib.rs

@@ -1975,9 +1975,7 @@ impl<'a> Resolver<'a> {
         if !module.no_implicit_prelude {
             if ns == TypeNS && self.extern_prelude.contains(&ident.name) {
                 if !self.session.features_untracked().extern_prelude {
-                    feature_err(&self.session.parse_sess, "extern_prelude",
-                                ident.span, GateIssue::Language,
-                                "access to extern crates through prelude is experimental").emit();
+                    return None;
                 }
 
                 let crate_id = self.crate_loader.process_path_extern(ident.name, ident.span);

src/test/run-make-fulldeps/extern-prelude/Makefile

@@ -7,6 +7,5 @@ all:
 	$(RUSTC) basic.rs --extern ep_lib=$(TMPDIR)/libep_lib.rlib
 	$(RUSTC) shadow-mod.rs --extern ep_lib=$(TMPDIR)/libep_lib.rlib
 	$(RUSTC) shadow-prelude.rs --extern Vec=$(TMPDIR)/libep_vec.rlib
-	$(RUSTC) feature-gate.rs --extern ep_lib=$(TMPDIR)/libep_lib.rlib 2>&1 | $(CGREP) "access to extern crates through prelude is experimental"
 	$(RUSTC) relative-only.rs --extern ep_lib=$(TMPDIR)/libep_lib.rlib 2>&1 | $(CGREP) "unresolved import"
 	$(RUSTC) relative-only.rs --extern ep_lib=$(TMPDIR)/libep_lib.rlib 2>&1 | $(CGREP) "failed to resolve"