track proc-macro expansions in the self-profiler

Use the proc-macro descr to track their individual expansions with
self-profiling events. This will help diagnose performance issues
with slow proc-macros.

compiler/rustc_expand/src/base.rs

@@ -1047,6 +1047,12 @@ impl<'a> ExtCtxt<'a> {
         self.current_expansion.id.expn_data().call_site
     }
 
+    /// Returns the current expansion kind's description.
+    pub(crate) fn expansion_descr(&self) -> String {
+        let expn_data = self.current_expansion.id.expn_data();
+        expn_data.kind.descr()
+    }
+
     /// Equivalent of `Span::def_site` from the proc macro API,
     /// except that the location is taken from the span passed as an argument.
     pub fn with_def_site_ctxt(&self, span: Span) -> Span {