Autogen: Clear sources meta data cache

Adds cmGeneratorTarget::clearSourcesCache() which clears the cache
of precomputed sources lists and object names. The cache gets
recomputed on demand.

Clearing the cache is necessary in case an OBJECT library
gets a source added after a target (ORIGIN) that includes it
computed it's external sources cache.
This may happen in AUTOMOC when ORIGIN is processed before the
OBJECT library which gets a mocs_compilation.cpp file added.

Closes #17152
Closes #17139
Closes #17085
Closes #15967

Source/cmGeneratorTarget.h

@@ -491,6 +491,11 @@ public:
   std::string GetOutputName(const std::string& config,
                             cmStateEnums::ArtifactType artifact) const;
 
+  /** Clears cached meta data for local and external source files.
+    * The meta data will be recomputed on demand.
+    */
+  void ClearSourcesCache();
+
   void AddSource(const std::string& src);
   void AddTracedSources(std::vector<std::string> const& srcs);