This CL adds support for:
https://discourse.wicg.io/t/allow-const-global-variables/684

It allows global const variables to be used as if they were numeric
literals. For example:

  const f0 = fround(0);
  ...
  function foo() {
    var v = f0;  // no type annotation.
    ...
    return f0;  // no return type annotation.
  }

BUG= https://bugs.chromium.org/p/v8/issues/detail?id=5540

Review-Url: https://chromiumcodereview.appspot.com/2435823002
Cr-Commit-Position: refs/heads/master@{#40477}