Improve creation of RelationalCommandCache expression in RelationalShapedQueryCompilingExpressionVisitor

Create just empty `new HashSet(StringComparer.Ordinal)` and in 2nd step dynamically add the initializers as in `GenerateRelationalCommandCacheExpression`.

Original code:
```csharp
var commandLiftableConstant = RelationalDependencies.RelationalLiftableConstantFactory.CreateLiftableConstant(
relationalCommandCache,
c => new RelationalCommandCache(
c.Dependencies.MemoryCache,
c.RelationalDependencies.QuerySqlGeneratorFactory,
c.RelationalDependencies.RelationalParameterBasedSqlProcessorFactory,
queryExpression,
_useRelationalNulls,
),
"relationalCommandCache",
typeof(RelationalCommandCache));
```