Parameter null checking needs to ensure testing for use in delegate parameters

[jaredpar]

The following behavior is not allowed

delegate void D(string x!!);

Need to ensure tests are written to verify this (current behavior is correct).

[jaredpar]

This test exists so closing

roslyn/src/Compilers/CSharp/Test/Semantic/Semantics/NullCheckedParameterTests.cs

Lines 21 to 33 in e5e5d21

	public void NullCheckedDelegateDeclaration()
	{
	var source = @"
	delegate void Del(string x!!, int y);
	class C
	{
	Del d = delegate(string k!!, int j) { /* ... */ };
	}";
	CreateCompilation(source, parseOptions: TestOptions.RegularPreview).VerifyDiagnostics(
	// (2,26): error CS8714: Parameter 'x' can only have exclamation-point null checking in implementation methods.
	// delegate void Del(int x!!, int y);
	Diagnostic(ErrorCode.ERR_MustNullCheckInImplementation, "x").WithArguments("x").WithLocation(2, 26));
	}