Mark constants as _INLINE_VAR constexpr.

[StephanTLavavej]

@cdacamar explained why he's implementing a new compiler warning:

I have been tightening up c1xx's modules implementation by making the compiler more strict and correct when it comes to assigning out linkage to entities emitted into associated .objs.

This warning will complain about non-inline constants in headers. We're almost completely consistent about using inline in C++17-and-later (or _INLINE_VAR in potentially-C++14), but Cameron found our inconsistent occurrences:

C:\msvc\binaries\x86chk\inc\vector(2113): warning C5260: the constant variable 'std::_VBITS' has internal linkage in an included header file context, but external linkage in imported header unit context; consider declaring it 'inline' as well if it will be shared across translation units, or 'static' to express intent to use it local to this translation unit
C:\msvc\binaries\x86chk\inc\regex(1166): warning C5260: the constant variable 'std::_Buf_incr' has internal linkage in an included header file context, but external linkage in imported header unit context; consider declaring it 'inline' as well if it will be shared across translation units, or 'static' to express intent to use it local to this translation unit
C:\msvc\binaries\x86chk\inc\functional(725): warning C5260: the constant variable 'std::_Space_size' has internal linkage in an included header file context, but external linkage in imported header unit context; consider declaring it 'inline' as well if it will be shared across translation units, or 'static' to express intent to use it local to this translation unit
C:\msvc\binaries\x86chk\inc\regex(1158): warning C5260: the constant variable 'std::_BRE_MAX_GRP' has internal linkage in an included header file context, but external linkage in imported header unit context; consider declaring it 'inline' as well if it will be shared across translation units, or 'static' to express intent to use it local to this translation unit
C:\msvc\binaries\x86chk\inc\regex(1160): warning C5260: the constant variable 'std::_Bmp_max' has internal linkage in an included header file context, but external linkage in imported header unit context; consider declaring it 'inline' as well if it will be shared across translation units, or 'static' to express intent to use it local to this translation unit
C:\msvc\binaries\x86chk\inc\regex(1161): warning C5260: the constant variable 'std::_Bmp_shift' has internal linkage in an included header file context, but external linkage in imported header unit context; consider declaring it 'inline' as well if it will be shared across translation units, or 'static' to express intent to use it local to this translation unit
C:\msvc\binaries\x86chk\inc\regex(1162): warning C5260: the constant variable 'std::_Bmp_chrs' has internal linkage in an included header file context, but external linkage in imported header unit context; consider declaring it 'inline' as well if it will be shared across translation units, or 'static' to express intent to use it local to this translation unit
C:\msvc\binaries\x86chk\inc\regex(1163): warning C5260: the constant variable 'std::_Bmp_mask' has internal linkage in an included header file context, but external linkage in imported header unit context; consider declaring it 'inline' as well if it will be shared across translation units, or 'static' to express intent to use it local to this translation unit
C:\msvc\binaries\x86chk\inc\regex(1164): warning C5260: the constant variable 'std::_Bmp_size' has internal linkage in an included header file context, but external linkage in imported header unit context; consider declaring it 'inline' as well if it will be shared across translation units, or 'static' to express intent to use it local to this translation unit
C:\msvc\binaries\x86chk\inc\regex(1167): warning C5260: the constant variable 'std::_ARRAY_THRESHOLD' has internal linkage in an included header file context, but external linkage in imported header unit context; consider declaring it 'inline' as well if it will be shared across translation units, or 'static' to express intent to use it local to this translation unit

• This fixes all of these occurrences, plus another that I found, _Size_after_ebco_v in <xstring>.
  • I need to detach the comment there due to wrapping.
• All are potentially C++14, so they need to use _INLINE_VAR.
• In <regex>, I'm upgrading const to constexpr.
• In <vector>, I'm adding a newline to avoid = alignment, and because typedefs and constants are different.

[StephanTLavavej]

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