Introspect default expressions as DefaultExpression

[morozov]

Fixes #7274.

This is a backport of the corresponding logic from #7197.

The idea is that AbstractSchemaManager implementations should not invoke AbstractPlatform::getCurrent*SQL(), the return value of which would match the expression string and trigger a deprecation. Instead, they should instantiate the corresponding DefaultExpression instance.

I haven't found an existing test where I could put a "no deprecation triggered" assertion, and I don't feel like writing a throwaway one. This is how the the changes can be formally verified:

➜ git grep -- '->getCurrent.*SQL' 4.4.x src/Schema/**/*SchemaManager.php
4.4.x:src/Schema/MySQLSchemaManager.php:            'current_timestamp()' => $platform->getCurrentTimestampSQL(),
4.4.x:src/Schema/MySQLSchemaManager.php:            'curdate()' => $platform->getCurrentDateSQL(),
4.4.x:src/Schema/MySQLSchemaManager.php:            'curtime()' => $platform->getCurrentTimeSQL(),
4.4.x:src/Schema/SQLServerSchemaManager.php:            return $this->platform->getCurrentTimestampSQL();

➜ git grep -- '->getCurrent.*SQL' HEAD src/Schema/**/*SchemaManager.php || echo Not found
Not found

[morozov]

The test failures are being fixed in #7276.