STL: Consider consistently rephrasing Standard mode diagnostics

[StephanTLavavej]

When a header for a newer C++ Standard is compiled with an older Standard mode, it emits a diagnostic. Our phrasing varies; some talk about the contents of the header:

STL/stl/inc/charconv

Line 3101 in 28ec9a3

#pragma message("The contents of <charconv> are only available with C++17 or later.")

Others talk about the main class or class template defined by that header:

STL/stl/inc/any

Line 445 in 28ec9a3

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

STL/stl/inc/variant

Line 1754 in 28ec9a3

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

Some don't bother to say "class" (and ignore the distinction between the basic_string_view class template and the string_view typedef):

STL/stl/inc/string_view

Line 15 in 28ec9a3

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

And some have custom phrasing:

STL/stl/inc/execution

Line 5141 in 28ec9a3

#pragma message("Parallel algorithms are only available with C++17 or later.")

Should we be consistent about this phrasing?

[StephanTLavavej]

My preference: "The contents of <meow> are only available with C++MEOW or later." This is currently used by <charconv>, <compare>, <concepts>, <filesystem>, <memory_resource>, and <ranges>.

[CaseyCarter]

Didn't @BillyONeal suggest that we should invert the conditionals that guard these "downlevel" diagnostics so the #pragma is nearer the top of the header file? (Admittedly these are separable issues, but the level of "creep" from combining them seems small to me.)

[SuperWig]

There's code inconsistencies too.

C++20 vs _HAS_CXX20 / ! vs not

STL/stl/inc/compare

Line 478 in 3b0a1c9

#else // ^^^ C++20 / not C++20 vvv

STL/stl/inc/ranges

Line 37 in 3b0a1c9

#else // ^^^ Concepts support / no Concepts vvv

STL/stl/inc/concepts

Line 367 in 3b0a1c9

#else // ^^^ <concepts> supported / <concepts> not supported vvv

Double sided arrows

STL/stl/inc/variant

Line 1753 in 3b0a1c9

#else // ^^^ _HAS_CXX17 ^^^ / vvv !_HAS_CXX17 vvv