Remember names of cfg-ed out items to mention them in diagnostics

`#[cfg]`s are frequently used to gate crate content behind cargo
features. This can lead to very confusing errors when features are
missing. For example, `serde` doesn't have the `derive` feature by
default. Therefore, `serde::Serialize` fails to resolve with a generic
error, even though the macro is present in the docs.

This commit adds a list of all stripped item names to metadata. This is
filled during macro expansion and then, through a fed query, persisted
in metadata. The downstream resolver can then access the metadata to
look at possible candidates for mentioning in the errors.

This slightly increases metadata (800k->809k for the feature-heavy
windows crate), but not enough to really matter.

compiler/rustc_ast/src/expand/mod.rs

@@ -1,3 +1,20 @@
 //! Definitions shared by macros / syntax extensions and e.g. `rustc_middle`.
 
+use rustc_span::{def_id::DefId, symbol::Ident};
+
+use crate::MetaItem;
+
 pub mod allocator;
+
+#[derive(Debug, Clone, Encodable, Decodable, HashStable_Generic)]
+pub struct StrippedCfgItem<ModId = DefId> {
+    pub parent_module: ModId,
+    pub name: Ident,
+    pub cfg: MetaItem,
+}
+
+impl<ModId> StrippedCfgItem<ModId> {
+    pub fn map_mod_id<New>(self, f: impl FnOnce(ModId) -> New) -> StrippedCfgItem<New> {
+        StrippedCfgItem { parent_module: f(self.parent_module), name: self.name, cfg: self.cfg }
+    }
+}