resolve: Don't speculatively load crates if this is a speculative res…

…olution

This avoids a rare rustdoc bug where loading `core` twice caused a
'cannot find a built-in macro' error:

1. `x.py build --stage 1` builds the standard library and creates a sysroot
2. `cargo doc` does something like `cargo check` to create `rmeta`s for all the crates (unrelated to what was built above)
3. the `cargo check`-like `libcore-*.rmeta` is loaded as a transitive dependency *and claims ownership* of builtin macros
4. `rustdoc` later tries to resolve some path in a doc link
5. suggestion logic fires and loads "extern prelude" crates by name
6. the sysroot `libcore-*.rlib` is loaded and *fails to claim ownership* of builtin macros

This fixes step 5. by not running suggestion logic if this is a
speculative resolution. Additionally, it marks `resolve_ast_path` as a
speculative resolution.

src/librustc_resolve/lib.rs

@@ -2385,8 +2385,12 @@ impl<'a> Resolver<'a> {
                             Res::Def(DefKind::Mod, _) => true,
                             _ => false,
                         };
-                        let mut candidates =
-                            self.lookup_import_candidates(ident, TypeNS, parent_scope, is_mod);
+                        // Don't look up import candidates if this is a speculative resolve
+                        let mut candidates = if record_used {
+                            self.lookup_import_candidates(ident, TypeNS, parent_scope, is_mod)
+                        } else {
+                            Vec::new()
+                        };
                         candidates.sort_by_cached_key(|c| {
                             (c.path.segments.len(), pprust::path_to_string(&c.path))
                         });
@@ -3200,7 +3204,7 @@ impl<'a> Resolver<'a> {
             &Segment::from_path(path),
             Some(ns),
             parent_scope,
-            true,
+            false,
             path.span,
             CrateLint::No,
         ) {