Moved warning stuff into MWDebug class.

Moved warning stuff into MWDebug class.

- Group common code in one instead of doing checks one
  time in GlobalFunctions.php and another time in Debug.php
- Remove the code catching the fact that a warning is due
  to a deprecation warning, no longer needed
- Pass the caller offset from wfDeprecated() to
  MWDebug::deprecated(); this was breaking deprecation
  notices for global objects.
- Changed PHP error level for deprecation notices (when
  $wgDevelopmentWarnings is set to true) from E_USER_NOTICE
  to E_USER_DEPRECATED since we now require PHP 5.3+
- Added E_USER_DEPRECATED to wfSuppressWarnings() and
  removed the check for the E_DEPRECATED constant being set
  now that we require PHP 5.3+
- Fixed MWDebugTest by calling wfSuppressWarnings() and
  wfRestoreWarnings() in setUp() and tearDown()

Change-Id: I6810b57c90d384de55a2cf177047767cdb734f79