Stephen Kelly authored Sep 12, 2012 and Brad King committed Sep 18, 2012

Removing the Process() API and removing the parameters from the
constructor will allow cmGeneratorExpressions to be cached and evaluated
with multiple configs for example, such as when evaluating target
properties.  This requires the creation of a new compiled representation
of cmGeneratorExpression.  The cmListFileBacktrace remains in the
constructor so that we can record where a particular generator
expression appeared in the CMakeLists file.