Avoid GenFuture shim when compiling async constructs

Previously, async constructs would be lowered to "normal" generators,
with an additional `from_generator` / `GenFuture` shim in between to
convert from `Generator` to `Future`.

The compiler will now special-case these generators internally so that
async constructs will *directly* implement `Future` without the need
to go through the `from_generator` / `GenFuture` shim.

The primary motivation for this change was hiding this implementation
detail in stack traces and debuginfo, but it can in theory also help
the optimizer as there is less abstractions to see through.

compiler/rustc_resolve/src/late.rs

@@ -3912,7 +3912,7 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
                 visit::walk_expr(self, expr);
                 self.diagnostic_metadata.current_type_ascription.pop();
             }
-            // `async |x| ...` gets desugared to `|x| future_from_generator(|| ...)`, so we need to
+            // `async |x| ...` gets desugared to `|x| async {...}`, so we need to
             // resolve the arguments within the proper scopes so that usages of them inside the
             // closure are detected as upvars rather than normal closure arg usages.
             ExprKind::Closure(box ast::Closure {