Toolset update: VS 2019 16.11 Preview 1

[StephanTLavavej]

• Update the toolset to VS 2019 16.11 Preview 1.
• Update the VM OS to 21h1-ent-g2.
• Comment the Start-Sleep workaround as requested by @barcharcraz in Toolset update: VS 2019 16.10 Preview 4 #1920 (comment) .
• Update tests.py for /std:c++20.
  • This makes both /std:c++20 and /std:c++latest add the custom feature 'c++2a'. It appears that libcxx hasn't yet converted their test suite to distinguish c++2a (C++20) and c++2b (C++23). I expect that we'll need to change tests.py later, so that these compiler options activate different custom features.

[x86]
Testing Time: 2195.59s
  Skipped          :   412
  Unsupported      :  2073
  Passed           : 25465
  Expectedly Failed:   626

[x64]
Testing Time: 2096.81s
  Skipped          :   412
  Unsupported      :  4027
  Passed           : 23511
  Expectedly Failed:   626

[CaseyCarter]

It appears that libcxx hasn't yet converted their test suite

"Hasn't yet" as of the last time updated our LLVM submodule reference, to be precise.