Add cmGeneratorTarget to represent a target during generation

Some per-target information and logic is common to all generators.
Some of that information is currently stored in cmTarget but that
should be reserved for the configure step.  Create a class to hold
per-target information for generators.  On construction classify
sources from the target and store them in separate members.  This
classification is already implemented separately in each generator.

bootstrap

@@ -208,6 +208,7 @@ CMAKE_CXX_SOURCES="\
   cmExportInstallFileGenerator \
   cmInstallDirectoryGenerator \
   cmGeneratedFileStream \
+  cmGeneratorTarget \
   cmGeneratorExpression \
   cmGlobalGenerator \
   cmLocalGenerator \