[clang] [Clang][C++23] Implement P1774R8: Portable assumptions (PR #81014)

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Sirraide wrote:

CI on Windows seems to have had a stroke?
```
FAILED: cmTC_77a79.exe
cmd.exe /C "cd . && C:\BuildTools\Common7\IDE\CommonExtensions\Microsoft\CMake\CMake\bin\cmake.exe -E vs_link_exe --intdir=CMakeFiles\cmTC_77a79.dir --rc="C:\Program Files (x86)\Windows Kits\10\bin\10.0.19041.0\x64\rc.exe" --mt="C:\Program Files (x86)\Windows Kits\10\bin\10.0.19041.0\x64\mt.exe" --manifests  -- C:\BuildTools\VC\Tools\MSVC\14.29.30133\bin\Hostx64\x64\link.exe /nologo CMakeFiles\cmTC_77a79.dir\getErrc.cpp.obj  /out:cmTC_77a79.exe /implib:cmTC_77a79.lib /pdb:cmTC_77a79.pdb /version:0.0 /machine:x64  /INCREMENTAL:NO /subsystem:console  kernel32.lib user32.lib gdi32.lib winspool.lib shell32.lib ole32.lib oleaut32.lib uuid.lib comdlg32.lib advapi32.lib && cd ."
MT: command "C:\Program Files (x86)\Windows Kits\10\bin\10.0.19041.0\x64\mt.exe /nologo /manifest cmTC_77a79.exe.manifest /outputresource:cmTC_77a79.exe;#1" failed (exit code 0x1f) with the following output:
mt.exe : general error c101008d: Failed to write the updated manifest to the resource of file "cmTC_77a79.exe". Operation did not complete successfully because the file contains a virus or potentially unwanted software.
```

Because I candidly don’t quite see how my `assume` code would being recognised as a virus...

https://github.com/llvm/llvm-project/pull/81014