debug fixes

[dsyme]

I noticed a few problems while running VisualFSharp tools in debug mode on some F# scripts. These fixes don't affect the actual product behaviour

1. We are getting an assert in the MSBuild resolution while processing scripts. The problem is that old broken code to compute the target framework version is kicking in and evaluating to "v4.0" which is no longer even supported. This computation of target framework version is only relevant in secondary stages of processing scripts and should just be HighestInstalledFrameworkVersion

2. The Simulated MSBuild code was failing if the gac directory does not exist. This code is only used by FCS. and was only failing for the .NET Standard/Core version of FCS This is now protected bya Directory.Exists

[cartermp]

Some ass removal

[KevinRansom]

@dsyme

        /home/vsts/work/1/s/src/fsharp/CompileOps.fs(2792,9): error FS1182: The value 'ilGlobals' is unused [/home/vsts/work/1/s/fcs/FSharp.Compiler.Service/FSharp.Compiler.Service.fsproj]
2019-01-31T13:48:56.1556589Z 

[KevinRansom]

Once tests are passing this is fine.