P2Ps should be allowed when ReferenceOutputAssembly=false even given TFM incompatibilities

_From @AArnott on March 3, 2017 18:40_

With VS2015 projects, I can have a P2P from a portable library to a net46 library by setting metadata on the project reference:

```xml

false

```

But with the .NET SDK projects, even with this metadata the build fails:

> C:\Program Files (x86)\Microsoft Visual Studio\2017\d15rel\MSBuild\Sdks\Microsoft.NET.Sdk\build\Microsoft.NET.Sdk.Common.targets(73,5): error : Project 'C:\git\pinvoke\src\CodeGeneration\CodeGeneration.csproj'
targets '.NETFramework,Version=v4.6'. It cannot be referenced by a project that targets '.NETPortable,Version=v0.0,Profile=Profile92'. [C:\git\pinvoke\src\CodeGeneration\CodeGeneration.csproj]

This blocks scenarios where a P2P exists merely for the sake of ensuring build ordering but without the assembly reference. In my particular scenario, the referenced project provides a binary that the build of the portable library picks up for code generation purposes.

_Copied from original issue: dotnet/sdk#939_