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/VideoBackends/D3DCommon/Common.cpp

@@ -110,7 +110,7 @@ std::vector<std::string> GetAdapterNames()
     std::string name;
     DXGI_ADAPTER_DESC desc;
     if (SUCCEEDED(adapter->GetDesc(&desc)))
-      name = UTF16ToUTF8(desc.Description);
+      name = WStringToUTF8(desc.Description);
 
     adapters.push_back(std::move(name));
   }