cmGlobalGenerator: Add cache for realpath() results

Cache the results of `realpath()` system calls in `cmGlobalGenerator`
to avoid repeating such calls for the same paths over and over.

Source/cmGlobalGenerator.cxx

@@ -7,6 +7,7 @@
 #include <cstdio>
 #include <cstdlib>
 #include <cstring>
+#include <functional>
 #include <initializer_list>
 #include <iterator>
 #include <sstream>
@@ -3110,6 +3111,16 @@ cmGlobalGenerator::GetFilenameTargetDepends(cmSourceFile* sf) const
   return this->FilenameTargetDepends[sf];
 }
 
+const std::string& cmGlobalGenerator::GetRealPath(const std::string& dir)
+{
+  auto i = this->RealPaths.lower_bound(dir);
+  if (i == this->RealPaths.end() ||
+      this->RealPaths.key_comp()(dir, i->first)) {
+    i = this->RealPaths.emplace_hint(i, dir, cmSystemTools::GetRealPath(dir));
+  }
+  return i->second;
+}
+
 void cmGlobalGenerator::ProcessEvaluationFiles()
 {
   std::vector<std::string> generatedFiles;

Source/cmGlobalGenerator.h

@@ -485,6 +485,8 @@ public:
     configs.emplace_back("$<CONFIG>");
   }
 
+  std::string const& GetRealPath(std::string const& dir);
+
 protected:
   // for a project collect all its targets by following depend
   // information, and also collect all the targets
@@ -676,6 +678,8 @@ private:
   mutable std::map<cmSourceFile*, std::set<cmGeneratorTarget const*>>
     FilenameTargetDepends;
 
+  std::map<std::string, std::string> RealPaths;
+
 #if !defined(CMAKE_BOOTSTRAP)
   // Pool of file locks
   cmFileLockPool FileLockPool;

Source/cmLocalGenerator.cxx

@@ -1177,7 +1177,7 @@ std::vector<BT<std::string>> cmLocalGenerator::GetIncludeDirectoriesImplicit(
     }
 
     for (std::string const& i : impDirVec) {
-      if (implicitSet.insert(cmSystemTools::GetRealPath(i)).second) {
+      if (implicitSet.insert(this->GlobalGenerator->GetRealPath(i)).second) {
         implicitDirs.emplace_back(i);
       }
     }
@@ -1188,7 +1188,7 @@ std::vector<BT<std::string>> cmLocalGenerator::GetIncludeDirectoriesImplicit(
                       &lang](std::string const& dir) {
     return (
       // Do not exclude directories that are not in an excluded set.
-      ((!cmContains(implicitSet, cmSystemTools::GetRealPath(dir))) &&
+      ((!cmContains(implicitSet, this->GlobalGenerator->GetRealPath(dir))) &&
        (!cmContains(implicitExclude, dir)))
       // Do not exclude entries of the CPATH environment variable even though
       // they are implicitly searched by the compiler.  They are meant to be