Fix isIntSafe and isDoubleSafe not considering some QVariant types

In `qgsexpression.cpp`, the functions `isIntSafe` and `isDoubleSafe` does not test for `QVariant::UInt`, `QVariant::LongLong` or `QVariant::ULongLong`. This causes for instance the expression `"attribute_name" < somevalue` to fail if `"attribute_name"` is a `BIGINT` column of a PostGIS database.


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