Add ArgumentNullException.ThrowIfNull

[stephentoub]

Fixes #48573

We will wait to roll this out broadly across dotnet/runtime until the !! feature from C# is available. C# may be able to target this method now or in the future now that it exists, and we'll use it explicitly in places where we can't rely on the C# feature, e.g. where we need to do the checks in a different order from what the compiler would emit (but the 90% case should end up using the C# feature).

In this PR I've only replaced call sites that had their own ThrowIfNull/NotNull already being used.

cc: @jcouv, @bartonjs

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, to please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[jcouv]

@stephentoub Sorry if I'm missing some context. I'm confused by "We will wait to roll this out broadly across dotnet/runtime until the !! feature from C# is available. C# may be able to target this method now or in the future now that it exists [...]".

The notes I had from email thread were:

• Roslyn injects an internal ThrowIfNull(object argument, string argumentName) helper into every assembly that uses the feature, similar to how it injects helpers into assemblies for other purposes, like switching on strings (SharpLab) or use of nullability annotations (SharpLab)
• We are free to liberally use !! throughout dotnet/runtime to replace places we currently do if (arg == null) throw new ArgumentNullException(nameof(arg)); for argument checking.
• We can choose to also introduce a public ThrowIfNull helper for devs to use directly, but it’s unrelated to !!.

So the compiler will not be targeting this method.
I would have expected dotnet/runtime to use this method before the parameter!! feature from C# becomes available, not after.
Tagging @jaredpar since he's the champion for parameter!!.

[stephentoub]

@jcouv, sorry for the confusion. What I meant was, I'm not going to first replace all of the thousands of throw sites with a manual call to ThrowIfNull and then again change all of them to use !!; that's way too much churn. Instead, we won't call ThrowIfNull for them, and we'll just use the !! support when it's available. If in the future !! targets this method, great, but the expectation is that for the foreseeable future it won't (doing so could harm R2R code due to lack of cross-module inlining, so we likely wouldn't want to consider having the compiler target this public ThrowIfNull until that's addressed in the future). Even with !!, though, we expect there will still be places we'll use ThrowIfNull directly, but it'll be easier to see where those places should be after we've rolled out use of !! (or as part of doing so). For example, places where we do a null check on some argument but we do so after we check and throw for other conditions, we might still want to explicitly use ThrowIfNull to maintain ordering / compat.