Reapply fixed "Honour 'use-external-names' in FileManager"

Was r202442

There were two issues with the original patch that have now been fixed.
1. We were memset'ing over a FileEntry in a test case. After adding a
   std::string to FileEntry, this still happened to not break for me.
2. I didn't pass the FileManager into the new compiler instance in
   compileModule. This was hidden in some cases by the fact I didn't
   clear the module cache in the test.

Also, I changed the copy constructor for FileEntry, which was memcpy'ing
in a (now) unsafe way.

llvm-svn: 202539

clang/lib/Basic/FileSystemStatCache.cpp

@@ -32,6 +32,7 @@ void FileSystemStatCache::anchor() { }
 
 static void copyStatusToFileData(const vfs::Status &Status,
                                  FileData &Data) {
+  Data.Name = Status.getName();
   Data.Size = Status.getSize();
   Data.ModTime = Status.getLastModificationTime().toEpochTime();
   Data.UniqueID = Status.getUniqueID();