Fuse <xstddef> into <type_traits>

[frederick-vs-ja]

<xstddef> is only included by <type_traits> after #3623 and #3633, so perhaps it's time to fuse <xstddef> into <type_traits>.

I'm also changing or eliminating some comments in test files that mention <xstddef>.

[StephanTLavavej]

Looks good to me, thanks!

• This shouldn't have source-breaking impact unless someone was directly including <xstddef>.
• There's one remaining mention of xstddef in a function name (the test is compile-only so the function is never called):

  STL/tests/std/tests/VSO_0000000_instantiate_iterators_misc/test.compile.pass.cpp

  Line 1867 in 3c1f3d7

  void xstddef_test() {

  
  I'm fine with this, there's no real reason to rearrange the test code or even rename the function. Amusingly, this test also still mentions xtgmath, which I fused away in Fix #192: <cmath>: Fuse <xtgmath.h> #374:

  STL/tests/std/tests/VSO_0000000_instantiate_iterators_misc/test.compile.pass.cpp

  Line 1905 in 3c1f3d7

  void xtgmath_integral_test_impl() {

• ⚠️ MSVC-internal changes (to setup etc.) will be required when merging this PR, as usual.

[StephanTLavavej]

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

[StephanTLavavej]

Thanks for simplifying the codebase! ✅ ✨ 🐱

[KrisGeorges]

This shouldn't have source-breaking impact unless someone was directly including xstddef.

Yes, there was actually someone out there doing that, believe it or not. :)