resolve: Fix resolution of empty paths passed from rustdoc

compiler/rustc_resolve/src/lib.rs

@@ -3298,7 +3298,9 @@ impl<'a> Resolver<'a> {
             PathResult::NonModule(path_res) if path_res.unresolved_segments() == 0 => {
                 Some(path_res.base_res())
             }
-            PathResult::NonModule(..) | PathResult::Failed { .. } => None,
+            PathResult::Module(ModuleOrUniformRoot::ExternPrelude)
+            | PathResult::NonModule(..)
+            | PathResult::Failed { .. } => None,
             PathResult::Module(..) | PathResult::Indeterminate => unreachable!(),
         }
     }

src/test/rustdoc-ui/intra-doc/global-path.rs

@@ -0,0 +1,8 @@
+// Doc link path with empty prefix that resolves to "extern prelude" instead of a module.
+
+// check-pass
+// edition:2018
+
+/// [::Unresolved]
+//~^ WARN unresolved link to `::Unresolved`
+pub struct Item;

src/test/rustdoc-ui/intra-doc/global-path.stderr

@@ -0,0 +1,10 @@
+warning: unresolved link to `::Unresolved`
+  --> $DIR/global-path.rs:6:6
+   |
+LL | /// [::Unresolved]
+   |      ^^^^^^^^^^^^ no item named `` in scope
+   |
+   = note: `#[warn(rustdoc::broken_intra_doc_links)]` on by default
+
+warning: 1 warning emitted
+