FileUtil: Add a class for Exists/IsDirectory/GetSize

Some code was calling more than one of these functions in a row
(in particular, FileUtil.cpp itself did it a lot...), which is
a waste since it's possible to call stat a single time and then
read all three values from the stat struct. This commit adds a
File::FileInfo class that calls stat once on construction and
then lets Exists/IsDirectory/GetSize be executed very quickly.

The performance improvement mostly matters for functions that
can be handling a lot of files, such as File::ScanDirectoryTree.

I've also done some cleanup in code that uses these functions.
For instance, some code had checks like !Exists() || !IsDirectory(),
which is functionally equivalent to !IsDirectory(), and some
code was using File::GetSize even though there was an IOFile
object that the code could call GetSize on.

Source/Core/VideoCommon/TextureCacheBase.cpp

@@ -424,7 +424,7 @@ void TextureCacheBase::DumpTexture(TCacheEntry* entry, std::string basename, uns
   std::string szDir = File::GetUserPath(D_DUMPTEXTURES_IDX) + SConfig::GetInstance().GetGameID();
 
   // make sure that the directory exists
-  if (!File::Exists(szDir) || !File::IsDirectory(szDir))
+  if (!File::IsDirectory(szDir))
     File::CreateDir(szDir);
 
   if (level > 0)