Multi-targeting: Error CS2012 sometimes when AppendTargetFrameworkToOutputPath is set to false

I am using Visual Studio 15.8.5 and I have a multi-target project setup that sets the output path based on the target and build configuration (like `output_dir\\`). Since I want the target as the parent folder I need to set `AppendTargetFrameworkToOutputPath` to false. However this setting also causes the target framework part not to be appended to the `obj` folder resulting in CS2012 sometimes due to race condition.

### Steps to reproduce

Here is a sample project file to reproduce this issue:
```XML

net40;netstandard2.0
false



..\..\Builds\net40\R_U_Debug\
..\..\Builds\netstandard2.0\R_U_Debug\

..\..\Builds\net40\R_U_Release\
..\..\Builds\netstandard2.0\R_U_Release\

```
### Actual behavior
Here is the error I get sometimes when I try to build:
> Error CS2012 Cannot open 'D:\Work\MyProject\obj\R_U_Debug\MyProject.dll' for writing -- 'The process cannot access the file 'D:\Work\MyProject\obj\R_U_Debug\MyProject.dll' because it is being used by another process.'

### Expected behavior
I tried setting a separate obj folder using the `BaseIntermediateOuputPath` property but this didn't take effect.
```XML
obj\net40
obj\netstandard2.0
```
Perhaps the `AppendTargetFrameworkToOutputPath` property should not be apply to the intermediate obj folders.

Start with the sample multi-target project and inspect how AppendTargetFrameworkToOutputPath and BaseIntermediateOutputPath are applied to the output and obj paths. Reproduce a parallel build with net40 and netstandard2.0; done means each target has an independent intermediate path and the intermittent CS2012 error no longer occurs.

Written by the indexing model from the issue text.