<optional> <variant> etc. #pragma message for old standard mode should have STL#### prefix

[BillyONeal]

The Visual Studio IDE team would like to better surface cases when users try to use new language features in older language modes, as the IDE currently can't display the pragma messages we use, like:

STL/stl/inc/optional

Line 641 in b72c0a6

#pragma message("class template optional is only available with C++17 or later.")

They requested that we change that message to be of the form STLNNNN (where N are digits) just like the rest of the library-generated diagnostics.

[StephanTLavavej]

Related: #209 "STL: Consider cleaning up and numbering static_assert messages".