Upgrade/Install: Invalidate OPcache for PHP files during updates.

When files are copied into place, check whether opcode invalidation is available and attempt to invalidate to avoid unintended behavior or fatal errors from themes, plugins, or core.

Introduces `wp_opcache_invalidate()` to allow safe invalidation of PHP files from opcode cache, and a filter, `wp_opcache_invalidate_file` to override the behavior.

Replaces the existing calls to `opcache_invalidate()` in the plugin and theme editors to use the new function.

Thanks to jnylen0 for porting over a patch from ClassicPress that provided much of the approach for what is being committed.

Props nigro.simone, dd32, JasWSInc, szepe.viktor, swissspidy, JanR, asalce, Garavani, pavelevap, pputzer, GregLone, benoitchantre, jadonn, doc987, kraftbj, Krstarica, jnylen0, nextendweb, williampatton, ayeshrajans, joostdevalk, stevenkussmaul, boogah, jorbin, mikeschroder.
Fixes #36455, #50354.
Built from https://develop.svn.wordpress.org/trunk@48160


git-svn-id: https://core.svn.wordpress.org/trunk@47929 1a063a9b-81f0-0310-95a4-ce76da25c4cd