Use constants in AMP_Allowed_Tags_Generated

[schlessera]

Feature description

While trying to investigate the reason for #4465, I noticed that the constants being used within the protoascii files are reverted to meaningless scalar values in the generated PHP files.

The PHP files should make use of defined constants like the ones defined in AmpProject\Layout or similar, so that it becomes much easier to reason about the generated rules.

Example before (extract):

'amp-audio' => array(
array(
    'tag_spec' => array(
        'amp_layout' => array(
            'defines_default_height' => true,
            'defines_default_width' => true,
            'supported_layouts' => array(
                2,
                3,
                1,
            ),
        ),
    ),
),

Example after (extract):

'amp-audio' => array(
array(
    'tag_spec' => array(
        'amp_layout' => array(
            'defines_default_height' => true,
            'defines_default_width' => true,
            'supported_layouts' => array(
                Layout::FIXED,
                Layout::FIXED_HEIGHT,
                Layout::FILL,
            ),
        ),
    ),
),

---

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Acceptance criteria

• All constants defined in protoascii should translate to constants defined in PHP.

Implementation brief

QA testing instructions

Demo

Changelog entry

[westonruter]

Doing this would be complicated given the current spec generator, which takes a data structure and then just does var_export(). This may make most sense to tackle as part of #3875.

[westonruter]

Being worked on as part of #3730.

[westonruter]

Implemented in ampproject/amp-toolbox-php#100