Upgrade to PHPStan v0.12

Additionally, set the new value and get the old one in one ini_set() call.

By either defining their types or renaming to match parent class (or
interface).

Allowing us to use more advanced checks and have full generic support.

This also adds a baseline file, since we have new violations being
reported but don't want to address them right now.

Actualize the content of the .gitattributes file

Removed performance tests

1. Removed no longer existing entries
2. Added new entries
3. Sorted using `sort` (ignores case and the leading dot, same as `ls`)

Ensure the constructor arguments are passed to custom classes

Flatten source and test directory structure

Removed the `Dbal` prefix from the class names and moved them to the `Doctrine\Tests\DBAL` namespace.

Made some platform class names reflect the lowest server version they support

The `PostgreSqlPlatform`, `SQLAnywherePlatform` and `SQLServerPlatform` classes have been renamed to `PostgreSQL94Platform`, `SQLAnywhere16Platform` and `SQLServer2012Platform` respectively.

When deprecating an old platform version, we can deprecate and later remove a specific class. Otherwise, there's no way to express the deprecation on the class level and the upgrade notes contain sentences like: "`XYZPlatform` and `XYZKeywords` now represent XYZ (version)".

Backport removal of legacy platforms and drivers from master