fix xdebug coverage (part1): xdebug_get_code_coverage return value format for executed line

[xKerman]

This pull request is related to #1589 .
(I will submit another pull request for supporting XDEBUG_CC_UNUSED and XDEBUG_CC_DEAD_CODE later)

xdebug_get_code_coverage() return value format is described in https://xdebug.org/docs/code_coverage :

The returned values for each line are:

• 1: this line was executed
• -1: this line was not executed
• -2: this line did not have executable code on it

According to this description, xdebug_get_code_coverage() should return value like:

[
    "/path/to/foo.php" => [
          3 => 1,  // executed line 3
          5 => 1,  // executed line 5
    ]
]

However, hhvm's xdebug_get_code_coverage() currently returns value like:

[
    "/path/to/foo.php" => [
          3 => 2,  // executed line 3 (2 times)
          5 => 3,  // executed line 5 (3 times)
    ]
]

In this pull request, hhvm's xdebug_get_code_coverage() return value format for executed line is changed to follow description in https://xdebug.org/docs/code_coverage .

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[xKerman]

Thanks!