<__msvc_int128.hpp>: Backport 128-bit integer-class types to C++14 mode

[frederick-vs-ja]

Thanks to @MattStephanson for having done the most works in #3012. As mentioned in #3012, having 128-bit integer-like types in all modes is useful for random number generation.

This PR also provides a library solution for DevCom-879048 with literal suffixes (__i128 and __u128) support.

[frederick-vs-ja]

Sometimes some (possibly unrelated) tests run forever. Is this triggering some compiler bug?

[strega-nil-ms]

saw some stuff, but I need to go pass out again (think I'm getting sick)

[strega-nil-ms]

Other than that, looks really good! thanks!

[strega-nil-ms]

[StephanTLavavej]

Thanks, this looks good! I pushed various minor changes:

• Consistent preprocessor comments.
• Always use braces for control flow.
• Already constrained for is_integral_v.
• Don't need to qualify numeric_limits.
• Rename to _Int128_detail.
• Add newline between non-chained if-statements.
• Unnecessary parentheses.
• _NODISCARD UDLs.
• _NODISCARD helper functions.
• Test capital 0B.
• Test capital hexits.

[StephanTLavavej]

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

[StephanTLavavej]

I had to push two more commits to fix internal test failures:

• EDG found two missed occurrences of _ZERO_OR_NO_INIT (I'm not sure why this was only found internally, but yay)
• /clr:pure, the betrayer of hope, has notorious trouble with variable templates. I've converted the logic into struct specializations dispatching to a static member function.

[StephanTLavavej]

I had to push two more commits to fix internal test failures for /clr and /clr:pure on x64.

• /clr:pure lacks virtually all intrinsics, so as usual we must avoid using them. Additionally, to avoid headaches, I am also disabling intrinsics for CUDA and the Intel compiler.
• There is a silent bad codegen bug for plain /clr when asking the result of -6 multiplied by -2. I've added a workaround but haven't reduced/reported it.

[StephanTLavavej]

Thanks for bringing this Future Technology to the past! 🤖 ⌚ 🛰️