Introduce sym::dummy and Ident::dummy.

The idea is to identify cases of symbols/identifiers that are not
expected to be used. There isn't a perfectly sharp line between "dummy"
and "not dummy", but I think it's useful nonetheless.

compiler/rustc_hir/src/hir.rs

@@ -243,7 +243,7 @@ impl<'hir> PathSegment<'hir> {
     }
 
     pub fn invalid() -> Self {
-        Self::new(Ident::empty(), HirId::INVALID, Res::Err)
+        Self::new(Ident::dummy(), HirId::INVALID, Res::Err)
     }
 
     pub fn args(&self) -> &GenericArgs<'hir> {