This PR was merged into the 4.2 branch.

Discussion
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[Cache] work aroung PHP memory leak

| Q             | A
| ------------- | ---
| Branch?       | 4.2
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #32220
| License       | MIT
| Doc PR        | -

Works around  https://bugs.php.net/76982, as identified by @goetas in the linked issue.

Commits
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5d55b91 [Cache] work aroung PHP memory leak

…s-grekas)

This PR was merged into the 3.4 branch.

Discussion
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[Cache] fix memory leak when using PhpArrayAdapter

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       | Fix #34687
| License       | MIT
| Doc PR        | -

Thanks to @adrienfr, I've been able to understand what causes this massive memory leak when using `PhpArrayAdapter`:
![image](https://user-images.githubusercontent.com/243674/70262187-303b1b00-1794-11ea-9fcb-21ae29c31ff0.png)

When tests run, a new kernel is booted for each test case. This means a new instance of `PhpArrayAdapter` is created, which means it loads its state again and again using `include` for e.g. `annotations.php` in this example.

The first obvious thing is that we see this doing `compile::*`: this means PHP is parsing the same file again and again. But shouldn't opcache prevent this? Well, it's disabled by default because `opcache.enable_cli=0`. To prove the point, here is a comparison with the same tests run with `php -dopcache.enable_cli=1`. The comparison is swapped, but you'll get it:

![image](https://user-images.githubusercontent.com/243674/70262616-fb7b9380-1794-11ea-81c3-6fea0145a63b.png)

But that's not over: because of https://bugs.php.net/76982 (see #32236 also), we still have a memory leak when the included file contains closures. And this one does.

This PR fixes the issue by storing the return value of the include statement into a static property. This fits the caching model of `PhpArrayAdapter`: it's a read-only storage for system caches - i.e. its content is immutable.

Commits
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4194c4c [Cache] fix memory leak when using PhpArrayAdapter