async: Give predictable, reserved name to binding generated from .await expressions.

This name makes it to debuginfo and allows debuggers to identify such bindings and
their captured versions in suspended async fns.

compiler/rustc_span/src/symbol.rs

@@ -272,6 +272,7 @@ symbols! {
         __D,
         __H,
         __S,
+        __awaitee,
         __try_var,
         _d,
         _e,
@@ -1022,7 +1023,6 @@ symbols! {
         pattern_parentheses,
         phantom_data,
         pin,
-        pinned,
         platform_intrinsics,
         plugin,
         plugin_registrar,