Use constexpr is_sorted, add charconv tests, update comments

[StephanTLavavej]

These are unrelated small changes to separate files - they could be split into separate PRs but I don't believe that would add any value.

• Use constexpr is_sorted() in tools/parallelize/parallelize.cpp. It's been available for a while, we just forgot about this occurrence.
• Adjust <execution> comments to take advantage of hover cards in the GitHub Pull Requests And Issues extension for VSCode. (Show hover cards when issue numbers are followed by , or . vscode-pull-request-github#2085 has been fixed, but is not yet released.)
• Update the libcxx skip lists now that @fsb4000 and @AlexGuteniev have filed issues for the various STL bugs mentioned. 😻
  • Remove redundant "compiler bug" and "STL bug" comments.
    • Note that VS 2019 16.8 Preview 3 toolset update #1290 is about to be merged and will remove the skip for VSO-1035737 "constexpr pointers-by-reference"; to avoid a merge conflict I have simply not updated that line.
    • Similarly, @Arzaghi submitted <complex>: prevent invalid conversions #1294 for <complex>: invalid conversions allowed #1256; I have chosen to revert the changes to this line to avoid a pointless merge conflict (I am confident we can merge this fix in the near future).
  • Group <cmath>: signbit() misses overloads for integer types #519 with the STL bugs.
  • Filed <array>: array<const T, 0> allows fill() and swap() #1295 separately from <array>: std::array<T,0> doesn't compile - when type is not default constructible #942. (Notably, we should be able to fix the former without breaking ABI.)
  • Remove VSO citations when we have GH issues (which are linked).
• Add test cases for floating-point numbers that UCRT printf() has trouble with; <charconv> was never affected but let's test these just in case.

[CaseyCarter]

Approve-with-suggestions.