Various cleanups: Simplify ios_base and locale constants

[StephanTLavavej]

Background

Using static constexpr is what allows the major improvement of dropping out-of-line definitions - see #3381 which made that change to ios_base's base class _Iosb.

<xiosbase>

• The DLL's exports don't mention any of _Dummy_enum, _Dummy_enum_val, _Stdio, _Openmode, _Openmask, _Seekdir, _Seekbeg, _Seekcur, _Seekend, _Openprot.
• Move retained enums to the bottom of _Iosb with an improved comment.
  • There's an internal MaxMPDCompat tool that notices the enum types and enumerators. With significant effort I think I could update the checked-in CompatibilityData baseline, but for now this is the easiest cleanup.
• We can drop _Stdio entirely (it's not an enumerator).
• We can simplify the definitions of the meowfield constants (I verified that this doesn't change them).
• The _Seekdir enumerators were being used exactly once; we can simply hardcode their values.

<xlocale>

Simplify locale's category constants.

We need to retain _Locbase and comment it as TRANSITION, ABI because it affects sizeof(locale) due to MSVC's deficient EBCO. (On x64, sizeof(locale) is 16, but removing _Locbase would shrink it to 8.)

However, there appears to be nothing stopping us from moving the constants into locale, as depicted in the Standard. I've verified that the DLL's exports are unaffected, and they don't mention _Locbase at all. (They also don't mention the names of these constants, excluding std::collate, std::ctype, and std::messages which are different.)

I don't believe we need _PGLOBAL - it's rarely used in the STL, and never for new code. crtdefs.h defines it to be __declspec(process) for _M_CEE (i.e. /clr and /clr:pure). https://learn.microsoft.com/en-us/cpp/cpp/process?view=msvc-170 explains "When compiling with /clr, global and static variables are per-process by default and do not need to use __declspec(process)." so this doesn't affect /clr at all, which is what matters. (As for /clr:pure, it's vanishingly unlikely to matter - these are constants, so if multiple application domains start getting multiple copies, who cares?)

using category = int; is mandated by the Standard. We previously said int for the constants, but the Standard depicts category, so I'm following that now that the typedef is in scope.

Finally, I'm upgrading const to constexpr - this is not depicted in the Standard, but I believe it's more consistent with the static constexpr variables we use everywhere these days. Note that [constexpr.functions] forbids constexpr-strengthening functions, but it doesn't care about variables.

[StephanTLavavej]

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

[StephanTLavavej]

I had to push a change to retain the enums in _Iosb, but at least I understand why now.