Common: Rename UTF16ToUTF8

This function does *not* always convert from UTF-16. It converts
from UTF-16 on Windows and UTF-32 on other operating systems.

Also renaming UTF8ToUTF16 for consistency, even though it
technically doesn't have the same problem since it only was
implemented on Windows.

Source/Core/Common/FileUtil.cpp

@@ -114,7 +114,7 @@ bool Exists(const std::string& path)
 bool IsDirectory(const std::string& path)
 {
 #ifdef _WIN32
-  return PathIsDirectory(UTF8ToUTF16(path).c_str());
+  return PathIsDirectory(UTF8ToWString(path).c_str());
 #else
   return FileInfo(path).IsDirectory();
 #endif