Existing single-parameter expressions work due to special casing
which assumes that if there is only one parameter, and the node
accepts arbitrary content, then the result is the input.

This is true for the existing expressions matching that
pattern - namely the "1" and "TARGET_NAME" expressions. However,
the LOWER_CASE, and UPPER_CASE expressions should also accept
arbitrary content, and in their case, the result is not the input.

Refactor the cmGeneratorExpressionEvaluator to allow that extension.
Actually evaluate the "1" and "0" nodes.