Use [NotNullWhen(true)] in more places

[stephentoub]

The issue fixed by #47579 inspired me to search for other places the consuming experience could be improved by using [NotNullWhen(true)], and found a lot. Most of these are overrides of object.Equals, but some are TryParse methods, other Equals overloads, and a smattering of other APIs. I essentially just did a quick regex search for methods that returned bool and took nullable reference type as the first input, and then added [NotNullWhen(true)] where it was obviously correct (I skipped over a few large areas, namely reflection, as this was already getting a bit out of hand).

For reference, the benefit of applying [NotNullWhen(true)] to a nullable argument is it lets the compiler know that if the method returns true then that argument wasn't null when the method was called, which lets you write code like:

if (string1.Equals(string2))
{
    Console.WriteLine(string2.Length); // no nullable warning because the compiler sees string2 is non-null
}

Note that for the most part I stayed away from modifying virtual methods, but obviously some of the overrides I modified aren't sealed, so technically this could be a breaking change (if someone derives from one of those types and overrides Equals and has nullable enabled and doesn't have [NotNullWhen(true)] on the argument). I can undo any folks feel queasy about.

cc: @jeffhandley, @krwq, @buyaa-n

[ghost]

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.

[ghost]

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.

[buyaa-n]

Left a few comment, overall looks good to me

[krwq]

LGTM with minor comment