Improve performance for Xdebug driver

Blacklist / whitelist filtering should be applied before sanitizing the coverage data obtained from Xdebug.

I moved the call to PHP_CodeCoverage_Driver_Xdebug::cleanup() to the position that seemed most reasonable to me.

This PR is related to https://github.com/sebastianbergmann/phpunit/issues/1187, as my personal example test suites now run 2 to 3 times faster than after upgrading to the latest stable releases of phpunit and phpunit-mock-objects and 4 to 6 times faster than before https://github.com/sebastianbergmann/phpunit-mock-objects/commit/c5e6274b8f2bf983cf883bb375cf44f99aff200e had been applied. :)