[10.x] expressions in migration computations fail

[tpetry]

Using an expression for a computation in any database currently fails.

// MySQL, MariaDB, SQLite 
Schema::table('products', function (Blueprint $table) {
  $table->string('discounted')->virtualAs(new Expression('price * 0.8'));
});

// MySQL, MariaDB, PostgreSQL, SQLite 
Schema::table('products', function (Blueprint $table) {
  $table->string('discounted')->storedAs(new Expression('price * 0.8'));
});

// MsSQL
Schema::table('products', function (Blueprint $table) {
  $table->computed('discounted', new Expression('price * 0.8'));
});

Error: Object of class Illuminate\Database\Query\Expression could not be converted to string

/home/runner/work/laravel-framework/laravel-framework/src/Illuminate/Database/Schema/Grammars/MySqlGrammar.php:1074
/home/runner/work/laravel-framework/laravel-framework/src/Illuminate/Database/Schema/Grammars/Grammar.php:223
/home/runner/work/laravel-framework/laravel-framework/src/Illuminate/Database/Schema/Grammars/Grammar.php:181
/home/runner/work/laravel-framework/laravel-framework/src/Illuminate/Database/Schema/Grammars/MySqlGrammar.php:213
/home/runner/work/laravel-framework/laravel-framework/src/Illuminate/Database/Schema/Blueprint.php:136
/home/runner/work/laravel-framework/laravel-framework/tests/Database/DatabaseMySqlSchemaGrammarTest.php:594

I've extended all grammars to correctly handle expressions for these migrations.