We weren't validating constraints on lifted operators, which allowed us to generate types such as `Nullable<int*>` in operator binding without reporting any errors. Fixes dotnet#56646.

…operators when lifting would cause invalid type arguments to be supplied to Nullable<T>, rather than erroring after creation. In addition, we now do this for lifted conversions as well, As a part of this, I found and corrected an implementation bug that caused code successfully emitted by the native compiler to emit bad code in Roslyn: we were treating pointer types like value types for the purposes of creating nullable value types, rather than treating them like reference types.

…-struct

* upstream/main: (829 commits)
  Re-enable BuildWithCommandLine test (dotnet#57584)
  Fix expected formatting in tests
  Simplify
  PR feedback
  Don't crash Use ExpressionBody on local functions in top level statements (dotnet#57571)
  Less linq
  Revert
  Update src/Features/VisualBasic/Portable/NavigationBar/VisualBasicNavigationBarItemService.vb
  Add test
  Simplify
  WIP
  Install .NET SDK in PR Validation pipeline
  Revert to VS2019 for PR Validation
  Update dependencies from https://github.com/dotnet/roslyn build 20211103.8
  Validate the checksum of source on disk (dotnet#57541)
  Clean up VsCodeWindowManager (dotnet#57582)
  Update VS editor package version
  Revert to VS2019 for official build
  Revert to VS2019 for official build
  StackFrame Parser/Lexer as EmbeddedLanguage (dotnet#56957)
  ...