cmLocalGenerator: Require a valid cmState::Snapshot in the ctor.

Refactor the local generator creation API to accept a
cmState::Snapshot.  Adjust MakeLocalGenerator to use the 'current'
snapshot in cases where there is no parent.  Create the snapshot
for subdirectories in cmMakefile::AddSubdirectory.

This means that snapshots are now created at the point of extending the tree,
as appropriate, and independently of the cmLocalGenerator and cmMakefile they
represent the state for.

Source/cmLocalVisualStudio10Generator.h

@@ -26,7 +26,8 @@ class cmLocalVisualStudio10Generator : public cmLocalVisualStudio7Generator
 public:
   ///! Set cache only and recurse to false by default.
   cmLocalVisualStudio10Generator(cmGlobalGenerator* gg,
-                                 cmLocalGenerator* parent);
+                                 cmLocalGenerator* parent,
+                                 cmState::Snapshot snapshot);
 
   virtual ~cmLocalVisualStudio10Generator();