cmGeneratorTarget: add a method to build classified command lines

This is essentially an extraction of the `Ninja` generator's command
line building logic. Porting generators to reuse this construction is a
task for the future.

Source/cmGeneratorTarget.h

@@ -517,6 +517,64 @@ public:
   std::vector<std::string> GetAppleArchs(std::string const& config,
                                          cm::optional<std::string> lang) const;
 
+  // The classification of the flag.
+  enum class FlagClassification
+  {
+    // The flag is for the execution of the tool (e.g., the compiler itself,
+    // any launchers, etc.).
+    ExecutionFlag,
+    // The flag is "baseline" and should be apply to TUs which may interact
+    // with this compilation (e.g., imported modules).
+    BaselineFlag,
+    // The flag is "private" and doesn't need to apply to interacting TUs.
+    PrivateFlag,
+    // Flags for the TU itself (e.g., output paths, dependency scanning, etc.).
+    LocationFlag,
+  };
+  enum class FlagKind
+  {
+    // Not a flag (executable or other entries).
+    NotAFlag,
+    // Flags for support of the build system.
+    BuildSystem,
+    // A compilation flag.
+    Compile,
+    // An include flag.
+    Include,
+    // A compile definition.
+    Definition,
+  };
+  struct ClassifiedFlag
+  {
+    ClassifiedFlag(FlagClassification cls, FlagKind kind, std::string flag)
+      : Classification(cls)
+      , Kind(kind)
+      , Flag(std::move(flag))
+    {
+    }
+
+    FlagClassification Classification;
+    FlagKind Kind;
+    std::string Flag;
+  };
+  using ClassifiedFlags = std::vector<ClassifiedFlag>;
+  ClassifiedFlags GetClassifiedFlagsForSource(cmSourceFile const* sf,
+                                              std::string const& config);
+  struct SourceVariables
+  {
+    std::string TargetPDB;
+    std::string TargetCompilePDB;
+    std::string ObjectDir;
+    std::string ObjectFileDir;
+    std::string DependencyFile;
+    std::string DependencyTarget;
+
+    // Dependency flags (if used)
+    std::string DependencyFlags;
+  };
+  SourceVariables GetSourceVariables(cmSourceFile const* sf,
+                                     std::string const& config);
+
   void AddExplicitLanguageFlags(std::string& flags,
                                 cmSourceFile const& sf) const;
 
@@ -1452,6 +1510,7 @@ private:
     std::map<std::string, cmFileSet const*> FileSetCache;
     std::map<cmGeneratorTarget const*, std::vector<cmGeneratorTarget const*>>
       SyntheticDeps;
+    std::map<cmSourceFile const*, ClassifiedFlags> SourceFlags;
   };
   mutable std::map<std::string, InfoByConfig> Configs;
 };