Overhaul constexpr <cstdlib> and <cinttypes>

[Stephan T. Lavavej (StephanTLavavej)]

Followup to #6434.

• Apply the extern "C++" inline namespace technique to __msvc_inttypes.hpp and __msvc_stdlib.hpp.
  • Reported by Cody Miller (@Codiferous). I forgot about these headers. The same technique is needed to avoid linker errors with UCRT div().
  • This also allows us to avoid the annoying "declare then #pragma function" dance.
• Avoid unnecessary function calls in abs() and div() overloads.
  • This is a pure cleanup to improve non-optimized debug mode codegen.

[Copilot]

Copilot review overview

🟡 Changes recommended

Mixed-mode linkage regression coverage is required for both headers.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Review tier: Balanced
Findings: 2

New issues introduced by this change (2)

Severity	Finding
	stl/​inc/​__msvc_inttypes.hpp — Please cover this linkage change with a mixed-translation-unit test that compiles <cinttypes>…
	stl/​inc/​__msvc_stdlib.hpp — Please add a mixed-translation-unit regression for this linkage change: compile one TU with…

What changed in this PR

Updates constexpr <cstdlib> and <cinttypes> functions to prevent UCRT linker conflicts and improve debug-mode code generation.

Changes:

• Places definitions in C++ inline namespaces.
• Implements integer abs() and div() overloads directly.

File	Description
stl/​inc/​__msvc_stdlib.hpp	Updates abs() and div() linkage and implementations; needs a mixed-translation-unit /Zc:cmath regression test.
stl/​inc/​__msvc_inttypes.hpp	Updates imaxabs() and imaxdiv() linkage; needs a mixed-translation-unit /Zc:cmath regression test.

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