Make deleting runfiles tree on Windows "best effort".

On Windows, we use hard links in runfiles tree, and we need
to delete and recreate all of them on every runfiles tree update
(otherwise the links might still point to outdated files).
Occasionally the hard link cannot be unlinked (due to permissions
or file being busy). This CL just ignores the error (and hopes for the best).
This will allow us to make progress on Windows.

Work towards #1212.

--
MOS_MIGRATED_REVID=121949474

src/main/tools/build-runfiles.cc

@@ -260,14 +260,17 @@ class RunfilesCreator {
       }
 
       FileInfoMap::iterator expected_it = manifest_.find(entry_path);
-      // When windows_compatible is enabled, if the hard link already existing
-      // is still
-      // in the mainifest, no need to recreate it.
-      // Note: here we assume the content won't change, which might not be true
-      // in some rare cases.
       if (expected_it == manifest_.end() ||
-          (!windows_compatible_ && expected_it->second != actual_info)) {
+          expected_it->second != actual_info) {
+#if !defined(__CYGWIN__)
         DelTree(entry_path, actual_info.type);
+#else
+        // On Windows, if deleting failed, lamely assume that
+        // the link points to the right place.
+        if (!DelTree(entry_path, actual_info.type)) {
+          manifest_.erase(expected_it);
+        }
+#endif
       } else {
         manifest_.erase(expected_it);
         if (actual_info.type == FILE_TYPE_DIRECTORY) {
@@ -386,12 +389,15 @@ class RunfilesCreator {
     }
   }
 
-  void DelTree(const std::string &path, FileType file_type) {
+  bool DelTree(const std::string &path, FileType file_type) {
     if (file_type != FILE_TYPE_DIRECTORY) {
       if (unlink(path.c_str()) != 0) {
+#if !defined(__CYGWIN__)
         PDIE("unlinking '%s'", path.c_str());
+#endif
+        return false;
       }
-      return;
+      return true;
     }
 
     EnsureDirReadAndWritePerms(path);
@@ -416,6 +422,7 @@ class RunfilesCreator {
     if (rmdir(path.c_str()) != 0) {
       PDIE("rmdir '%s'", path.c_str());
     }
+    return true;
   }
 
  private: