ToolLocationHelper.GetSupportedTargetFrameworks/GetPathToReferenceAssemblies are not aware of .NET Core

### Issue Description
ToolLocationHelper.GetSupportedTargetFrameworks and ToolLocationHelper.GetPathToReferenceAssemblies provide information about installed .NET Framework, UWP, and Silverlight frameworks, but not installed .NET Core frameworks.

### Steps to Reproduce
On machine with .NET Core 3.1 installed, call:

- ToolLocationHelper.GetSupportedTargetFrameworks();
- ToolLocationHelper.GetPathToReferenceAssemblies(new FrameworkName(".NETCoreApp", new Version(3, 1)));

### Expected Behavior
GetSupportedTargetFrameworks() returns a collection that includes .NETCoreApp,Version=v3.1.
GetPathToReferenceAssemblies returns a collection of paths to directories containing ,NET Core 3.1 reference assemblies.

### Actual Behavior
GetSupportedTargetFrameworks() returns a collection that does not include any .NETCoreApp framework names.
GetPathToReferenceAssemblies returns an empty collection.

### Versions & Configurations
Visual Studio 16.8.0 Preview 4.0 (main 30515.42)