[SPARK-12848][SQL] Change parsed decimal literal datatype from Double to Decimal

The current parser turns a decimal literal, for example ```12.1```, into a Double. The problem with this approach is that we convert an exact literal into a non-exact ```Double```. The PR changes this behavior, a Decimal literal is now converted into an extact ```BigDecimal```.

The behavior for scientific decimals, for example ```12.1e01```, is unchanged. This will be converted into a Double.

This PR replaces the ```BigDecimal``` literal by a ```Double``` literal, because the ```BigDecimal``` is the default now. You can use the double literal by appending a 'D' to the value, for instance: ```3.141527D```

cc davies rxin

Author: Herman van Hovell <hvanhovell@questtec.nl>

Closes #10796 from hvanhovell/SPARK-12848.