@covers broken with namespaced function

[Krinkle]

It seems @covers is unable to describe a function provided by a namespace.

Example: https://github.com/wikimedia/at-ease

src/Functions.php:

namespace MediaWiki;

function restoreWarnings() {
 ..
}

tests/FunctionsTest.php

class .. {
     /*
     * @covers MediaWiki\restoreWarnings
     */
  public function testWarningSuppression() {
    ..
  }
}

$ phpunit --coverage-html coverage/
PHPUnit 4.8.10 by Sebastian Bergmann and contributors.

Trying to @cover or @use not existing class or interface "\MediaWiki\suppressWarnings".

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Previously filed as #1797.