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/InputCommon/ControllerInterface/DInput/DInput.cpp

@@ -38,7 +38,7 @@ std::string GetDeviceName(const LPDIRECTINPUTDEVICE8 device)
   std::string result;
   if (SUCCEEDED(device->GetProperty(DIPROP_PRODUCTNAME, &str.diph)))
   {
-    result = StripSpaces(UTF16ToUTF8(str.wsz));
+    result = StripSpaces(WStringToUTF8(str.wsz));
   }
   else
   {