Treat "await" as an invalid identifier (#4954)

It is valid (outside `async` functions) in the "script" parse goal, but always invalid in the "module" parse goal.

Fixes #4952.

packages/babel-plugin-transform-es2015-function-name/test/fixtures/function-name/await/actual.js

@@ -0,0 +1,3 @@
+export {};
+
+var obj = { await: function () {} };

packages/babel-plugin-transform-es2015-function-name/test/fixtures/function-name/await/expected.js

@@ -0,0 +1,3 @@
+export {};
+
+var obj = { await: function _await() {} };