The SetsRequiredMembersAttribute prevents the compiler from checking the required member list of a type when calling that constructor, and suppresses any errors from a base type's list being invalid.

Specification: https://github.com/dotnet/csharplang/blob/main/proposals/required-members.md
Test plan: dotnet#57046

…operties should be considered initialized.

…o setsrequiredmembers

* upstream/features/required-members: (808 commits)
  Update for definite assignment changes
  Remove duplicate package references (dotnet#60658)
  Formatting and code generation options (dotnet#60127)
  Trim unnessasary leading lines when removing usings (dotnet#60672)
  Pass options to FixAllAsync, simplify CodeAction registration (dotnet#60665)
  Fallout
  Lint
  Restore CodeStyle test projects
  Update struct field definite assignment tests
  Global indentation options - take 2 (dotnet#60565)
  Run continuation to dispose of cancellation token source (dotnet#60653)
  Fixup
  Update tests
  Cleanup
  Cleanup
  move properties
  Delay starting the work to scan for todo-items
  Simplify
  Clean up syntax context
  Clean up syntax context
  ...

…n chaining to a constructor with the attribute. Add more tests.

…o setsrequiredmembers

* upstream/features/required-members: (66 commits)
  Fix dotnet#55183: Add SymbolVisitor<TArgument, TResult> (dotnet#56530)
  Simplifier options (dotnet#60174)
  Remove duplicated asset
  Do not try to refcount solution syncing when communicating with OOP
  Delay symbol-search index updating until solution is fully loaded.
  add more miscellaneous tests for checked operators (dotnet#60727)
  Support checked operators in explicit interface implementation (dotnet#60715)
  Avoid formatting diagnostics with raw strings (dotnet#60655)
  Make heading levels for warning waves documentation consistent (dotnet#60721)
  Clean up IDiagnosticService extension methods
  Remove #nullable enable
  Add integration test to flag MEF composition breaks
  Generate static abstract interface members correctly (dotnet#60618)
  Merge release/dev17.2 to main (dotnet#60682)
  Fix FAR on checked operators (dotnet#60698)
  Add implement interface support for checked operators and cast operators (dotnet#60719)
  Update csc.dll path in launch.json (dotnet#60663)
  Grab bag of UTF8 string support in IDE features (dotnet#60599)
  Allow code actions to retrieve options for any language (dotnet#60697)
  Fix flaky VSTypeScriptHandlerTests  (dotnet#60706)
  ...