Rename cmFileTimeComparison to cmFileTimeCache

The name `cmFileTimeCache` reflects the functionality of the class more
appropriately.

Source/cmFileTimeCache.cxx

@@ -0,0 +1,58 @@
+/* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+   file Copyright.txt or https://cmake.org/licensing for details.  */
+#include "cmFileTimeCache.h"
+
+#include <string>
+#include <unordered_map>
+#include <utility>
+
+cmFileTimeCache::cmFileTimeCache() = default;
+
+cmFileTimeCache::~cmFileTimeCache() = default;
+
+bool cmFileTimeCache::Load(std::string const& fileName, cmFileTime& fileTime)
+{
+  // Use the stored time if available.
+  {
+    auto fit = this->FileTimes.find(fileName);
+    if (fit != this->FileTimes.end()) {
+      fileTime = fit->second;
+      return true;
+    }
+  }
+  // Read file time from OS
+  if (!fileTime.Load(fileName)) {
+    return false;
+  }
+  // Store file time in cache
+  this->FileTimes[fileName] = fileTime;
+  return true;
+}
+
+bool cmFileTimeCache::FileTimeCompare(std::string const& f1,
+                                      std::string const& f2, int* result)
+{
+  // Get the modification time for each file.
+  cmFileTime ft1, ft2;
+  if (this->Load(f1, ft1) && this->Load(f2, ft2)) {
+    // Compare the two modification times.
+    *result = ft1.Compare(ft2);
+    return true;
+  }
+  // No comparison available.  Default to the same time.
+  *result = 0;
+  return false;
+}
+
+bool cmFileTimeCache::FileTimesDiffer(std::string const& f1,
+                                      std::string const& f2)
+{
+  // Get the modification time for each file.
+  cmFileTime ft1, ft2;
+  if (this->Load(f1, ft1) && this->Load(f2, ft2)) {
+    // Compare the two modification times.
+    return ft1.DifferS(ft2);
+  }
+  // No comparison available.  Default to different times.
+  return true;
+}