cmLocalGenerator: Use cached real paths for implicit include directories

Include directories requested by the user that are also implicit
include directories were resolved with an uncached real path lookup for
every target and configuration.  Use the cached lookup of the global
generator, as for the other include directories, and clear that cache
on every configure.

Source/cmGlobalGenerator.cxx

@@ -2604,6 +2604,7 @@ void cmGlobalGenerator::ClearGeneratorMembers()
   this->RuleHashes.clear();
   this->DirectoryContentMap.clear();
   this->ExistingSourceFiles.clear();
+  this->RealPaths.clear();
   this->XcFrameworkPListContentMap.clear();
   this->BinaryDirectories.clear();
   this->GeneratedFiles.clear();

Source/cmLocalGenerator.cxx

@@ -1544,7 +1544,8 @@ std::vector<BT<std::string>> cmLocalGenerator::GetIncludeDirectoriesImplicit(
   if (!stripImplicitDirs) {
     // Append implicit directories that were requested by the user only
     for (BT<std::string> const& udr : userDirs) {
-      if (cm::contains(implicitSet, cmSystemTools::GetRealPath(udr.Value))) {
+      if (cm::contains(implicitSet,
+                       this->GlobalGenerator->GetRealPath(udr.Value))) {
         emitBT(udr);
       }
     }