Remove MSBuild assemblies from the GAC at VSIX install time.

[rainersigwald]

Customers repeatedly hit problems caused by having stale MSBuild assemblies in their Global Assembly Cache.

For example,

• https://developercommunity.visualstudio.com/content/problem/208631/unable-to-create-or-open-solution.html
• https://developercommunity.visualstudio.com/content/problem/953754/after-update-to-1650-no-longer-able-to-loadcreate.html
• many more

We have long considered finding a way to ensure that MSBuild is not in the GAC when the package is updated, perhaps by running

gacutil /u "Microsoft.Build.Utilities.Core, Version=15.1.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a, processorArchitecture=MSIL"
gacutil /u "Microsoft.Build.Task.Core, Version=15.1.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a, processorArchitecture=MSIL"
gacutil /u "Microsoft.Build.Conversion.Core, Version=15.1.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a, processorArchitecture=MSIL"
gacutil /u "Microsoft.Build.Engine, Version=15.1.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a, processorArchitecture=MSIL"
gacutil /u "Microsoft.Build.Framework, Version=15.1.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a, processorArchitecture=MSIL"

in a related VS installer package.