<format>: delay vformat instantiation

[fsb4000]

Fixes #2329

[StephanTLavavej]

• Reported VSO-1433873 "Standard Library Header Units: Adding template <int = 0> to vformat() emits warnings C4265 and C4365".
  • Reported internally instead of on DevCom because my repro was partially reduced to "check out and build this branch in my fork of the STL" (which @cdacamar has expressed a preference for, but which DevCom triage presumably finds problematic).
• Adjusted preprocessor logic to #if defined(__clang__) || defined(__EDG__), which distinguishes Clang/EDG from C1XX.
  • In general, _MSC_VER is always defined (everyone imitates C1XX), so that doesn't indicate a particular front-end.
• Also adjusted the comments to our conventional // ^^^ no workaround / workaround vvv form.
• Extracted the workaround into a macro _TEMPLATE_INT_0_NODISCARD (later #undef'ed) because it needs to be repeated 4x, and clang-format otherwise produces ugly results.
• Added a comment improves throughput, see GH-2329.
  • In general, doing anything subtle that isn't required by the Standard is typically a good reason to have a comment. Mentioned briefly in the issue: "We should probably have comments as to why we're doing this."
• The workaround is repeated for all 4 overloads because the (string_view, format_args) overload isn't special - it was just the only one being tested in P1502R1_standard_library_header_units. Adding a wide string test case revealed that the compiler issue also affected (wstring_view, wformat_args). So, I'm permanently adding wide coverage. (And slightly enhancing the test case to stringify an int 1729, although that didn't reveal anything.) I didn't think that adding locale cases was necessary, but the workaround is almost certainly needed for them too, so I did that.
• I am not sure if the warning C4265 (non-virtual dtor) is valid - _Fmt_iterator_buffer certainly has virtual functions with a non-virtual dtor, but we never delete it through a pointer-to-base (it's always a local variable on the stack). However, the warning C4365 occurrences were actually valid - <format> was passing the result of pointer subtraction, which is ptrdiff_t, to _Count_separators() which takes size_t, so I'm adding static_casts:

  STL/stl/inc/format

  Line 1998 in 178b840

  _NODISCARD inline int _Count_separators(size_t _Digits, const string_view _Groups) {

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

Thanks for improving <format> throughput and finding the modules bug here (which has been fixed)! 🎉 📈 😻