Makefile,Ninja: De-duplicate ComputeObjectFilenames method

Move the method implementation up to `cmLocalCommonGenerator`
to avoid duplicating it in each generator.

Source/cmLocalCommonGenerator.h

@@ -5,6 +5,7 @@
 
 #include "cmConfigure.h" // IWYU pragma: keep
 
+#include <map>
 #include <string>
 
 #include "cmLocalGenerator.h"
@@ -12,6 +13,7 @@
 class cmGeneratorTarget;
 class cmGlobalGenerator;
 class cmMakefile;
+class cmSourceFile;
 
 /** \class cmLocalCommonGenerator
  * \brief Common infrastructure for Makefile and Ninja local generators.
@@ -30,6 +32,10 @@ public:
   std::string GetTargetFortranFlags(cmGeneratorTarget const* target,
                                     std::string const& config) override;
 
+  void ComputeObjectFilenames(
+    std::map<cmSourceFile const*, std::string>& mapping,
+    cmGeneratorTarget const* gt = nullptr) override;
+
 protected:
   std::string WorkingDirectory;