To fix ENG-6666, make NValue cast operations produce un-inlined values

I searched all the expression classes for calls to NValue's castAs method.  There were two: one was the cast operator expression, and the other was CASE WHEN.  This unit test was also failing in master for the same reason as cast expression.  (I'm guessing that the cast is needed for expressions like CASE ... WHEN x ELSE y END where x and y have different types.)