Fix thread-safety issues in MSBuild classes with upcoming multi-threaded node support

[AR-May]

Many MSBuild classes were not originally designed with multi-threaded execution in mind. With the introduction of multi-threaded nodes, we anticipate a number of race conditions and concurrency issues that will need to be addressed.

Below is a preliminary (incomplete) list of classes that are likely to require thread-safety fixes:

• BuildRequestEntry.cs
• TaskExecutionHost.cs
• Toolset.cs
• TaskFactoryWrapper.cs
• TaskRegistry.cs
• TypeLoader.cs
• NodeProviderOutOfProcTaskHost.cs

[AR-May]

In TypeLoader.cs static fields result in a race condition.

For many of the mentioned classes above refer to the prototype https://github.com/dotnet/msbuild/tree/dev/AR-May/multi-threaded-msbuild-3