resolve: Fix access to extern and stdlib prelude from opaque macros

Ok, it's hard to explain what happens, but identifier's hygienic contexts need to be "adjusted" to modules/scopes before they are resolved in them.
To be resolved in all kinds on preludes the identifier needs to be adjusted to the root expansion (aka "no expansion").

Previously this was done for the `macro m() { ::my_crate::foo }` case, but forgotten for all other cases.

src/librustc_resolve/lib.rs

@@ -2247,6 +2247,7 @@ fn resolve_ident_in_lexical_scope(&mut self,
         }
 
         if !module.no_implicit_prelude {
+            ident.span.adjust(Mark::root());
             if ns == TypeNS {
                 if let Some(binding) = self.extern_prelude_get(ident, !record_used) {
                     return Some(LexicalScopeBinding::Item(binding));

src/librustc_resolve/macros.rs

@@ -856,6 +856,7 @@ struct Flags: u8 {
                     match self.hygienic_lexical_parent(module, &mut ident.span) {
                         Some(parent_module) => WhereToResolve::Module(parent_module),
                         None => {
+                            ident.span.adjust(Mark::root());
                             use_prelude = !module.no_implicit_prelude;
                             match ns {
                                 TypeNS => WhereToResolve::ExternPrelude,

src/test/ui/hygiene/auxiliary/stdlib-prelude.rs

@@ -0,0 +1,3 @@
+#![feature(decl_macro)]
+
+pub macro stdlib_macro() {}

src/test/ui/hygiene/stdlib-prelude-from-opaque-early.rs

@@ -0,0 +1,21 @@
+// check-pass
+// aux-build:stdlib-prelude.rs
+
+#![feature(decl_macro)]
+#![feature(prelude_import)]
+
+extern crate stdlib_prelude;
+
+#[prelude_import]
+use stdlib_prelude::*;
+
+macro mac() {
+    mod m {
+        use std::mem; // OK (extern prelude)
+        stdlib_macro!(); // OK (stdlib prelude)
+    }
+}
+
+mac!();
+
+fn main() {}

src/test/ui/hygiene/stdlib-prelude-from-opaque-late.rs

@@ -0,0 +1,16 @@
+// check-pass
+
+#![feature(decl_macro)]
+
+macro mac() {
+    mod m {
+        fn f() {
+            std::mem::drop(0); // OK (extern prelude)
+            drop(0); // OK (stdlib prelude)
+        }
+    }
+}
+
+mac!();
+
+fn main() {}