Test plan for "nested stackalloc"

[gafter]

This is a placeholder for a test plan for #26759 where we permit stackalloc expressions in positions other than those to which they were restricted in C# 7.3.

• Specification checked in to csharplang. (See Champion "Permit stackalloc in nested expressions" (16.3, Core 3) csharplang#1412)
• Test the IDE version-upgrade fixer
• Add some tests for generated code:
  • nested expressions
  • ctor-initializer
  • field initializer
  • interactive
  • auto-property initializer
• Tests that demonstrate that we compute the escape scope correctly

[gafter]

Need a test something like this:

        [Fact]
        [WorkItem(13590, "https://github.com/dotnet/roslyn/issues/13590")]
        public void RefStructVariable()
        {
            string source =
@"public ref struct RS
{
    public static implicit operator RS(Span<int> span) => new RS();
}
RS x = new stackalloc[100]; // error: escape into field
";
            var tree = Parse(source, options: TestOptions.Script);
            var compilation = CreateCompilationWithMscorlib45(new[] { tree });

            compilation.VerifyDiagnostics(
// some appropriate error
                );
        }

[gafter]

The test suggested just above isn't a scenario because you can't declare a variable of a ref struct type at the top level in a script. Such a variable would become a field and it is an error.