ENH: Merging changes from branch CMake-SourceFile2-b between tags

CMake-SourceFile2-bp and CMake-SourceFile2-b-mp1 to trunk.  This
commit is surrounded by tags CMake-SourceFile2-b-mp1-pre and
CMake-SourceFile2-b-mp1-post on the trunk.

The changes re-implement cmSourceFile and the use of it to allow
instances to be created much earlier.  The use of cmSourceFileLocation
allows locating a source file referenced by a user to be much simpler
and more robust.  The two SetName methods are no longer needed so some
duplicate code has been removed.  The strange "SourceName" stuff is
gone.  Code that created cmSourceFile instances on the stack and then
sent them to cmMakefile::AddSource has been simplified and converted
to getting cmSourceFile instances from cmMakefile.  The CPluginAPI has
preserved the old API through a compatibility interface.

Source lists are gone.  Targets now get real instances of cmSourceFile
right away instead of storing a list of strings until the final pass.

TraceVSDependencies has been re-written to avoid the use of
SourceName.  It is now called TraceDependencies since it is not just
for VS.  It is now implemented with a helper object which makes the
code simpler.

Source/cmLocalXCodeGenerator.h

@@ -35,8 +35,6 @@ public:
   void GetTargetObjectFileDirectories(cmTarget* target,
                                       std::vector<std::string>& 
                                       dirs);
-  // return the source name for the object file
-  virtual std::string GetSourceObjectName(cmSourceFile& );
 private:
 
 };