Rename and reorder lots of lifetimes.

- Replace non-standard names like 's, 'p, 'rg, 'ck, 'parent, 'this, and
  'me with vanilla 'a. These are cases where the original name isn't
  really any more informative than 'a.
- Replace names like 'cx, 'mir, and 'body with vanilla 'a when the lifetime
  applies to multiple fields and so the original lifetime name isn't
  really accurate.
- Put 'tcx last in lifetime lists, and 'a before 'b.

compiler/rustc_builtin_macros/src/global_allocator.rs

@@ -65,7 +65,7 @@ struct AllocFnFactory<'a, 'b> {
     span: Span,
     ty_span: Span,
     global: Ident,
-    cx: &'b ExtCtxt<'a>,
+    cx: &'a ExtCtxt<'b>,
 }
 
 impl AllocFnFactory<'_, '_> {