Code coverage requires DebugType full

For projects using the roslyn based project system, coverage requires `DebugType = Full` as shown below:

```xml
  <PropertyGroup>
    <TargetFramework>netcoreapp1.1</TargetFramework> 
    <DebugType>Full</DebugType>
  </PropertyGroup>
```

This issue tracks the requirement to support portable symbols in coverage infrastructure.