Enable regex generator nullable reference types validation

[stephentoub]

Multiple times in the past we've audited the regex source generator for nullable reference type annotations, but as it was only compiled officially for netstandard2.0, any work we did to get it green eventually rotted. This follows the work done to enable also building a .NET Core target for the json generator to do the same for the regex generator. In doing so, I also took the opportunity to clean up the REGEXGENERATOR compilation constant.

[ghost]

Tagging subscribers to this area: @dotnet/area-system-text-regularexpressions
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Multiple times in the past we've audited the regex source generator for nullable reference type annotations, but as it was only compiled officially for netstandard2.0, any work we did to get it green eventually rotted. This follows the work done to enable also building a .NET Core target for the json generator to do the same for the regex generator. In doing so, I also took the opportunity to clean up the REGEXGENERATOR compilation constant.

Author:	stephentoub
Assignees:	-
Labels:	area-System.Text.RegularExpressions
Milestone:	8.0.0

[stephentoub]

@ViktorHofer, this currently fails with:
"CSC : error CS8032: An instance of analyzer System.Text.RegularExpressions.Generator.RegexGenerator cannot be created from D:\repos\runtime\artifacts\bin\System.Text.RegularExpressions.Generator\Debug\net7.0\System.Text.RegularExpressions.Generator.dll : Could not load
file or assembly 'System.Runtime, Version=7.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'. The system cannot find the file specified.. [D:\repos\runtime\src\libraries\System.Private.Xml\src\System.Private.Xml.csproj::TargetFramework=net8.0-windows]"
Any suggestion before I dig in deeper?

[joperezr]

Other than the small comments, changes LGTM.