Respect EnableUnmanagedDebugging property to enable native debugging

**Visual Studio Version**: 16.7 P1

**Summary**:
The `EnableUnmanagedDebugging` property isn't honored for .NET Core projects and launchSettings.json files need to be created just for the sake of enable native debugging: https://github.com/dotnet/project-system/issues/1125#issuecomment-442289060.

**Steps to Reproduce**:

1. Create a .NET Core project which launches a sub process in which you want to set a breakpoint.

2. Add the `EnableUnmanagedDebugging` property and set it to true.

3. F5 the project and notice that the module isn't loaded and the breakpoint isn't hit.

**Expected Behavior**:
Setting the `EnableUnmanagedDebugging` property should be honored by the project system.
```

net5.0
true

...

```
**Actual Behavior**:
The property isn't honored and a launchSettings.json needs to be created (either manually or by checking the "Enable native code debugging" checkbox in the Debug pane).

**User Impact**:
launchSettings.json files need to be checked in for every project to honor that setting or need to be created on the fly. For projects like dontet/runtime the former is not doable because of the huge amount of projects and the latter is cumbersome as the launchSettings file needs to be placed in the Properties folder which is a candidate for `git clean -xdf`.

cc @davkean