Enable JIT tracking for portable PDB debugging (#2899)
Enable JIT tracking for portable PDB debugging (#2899)

Setting JIT tracking to true adds the `DebuggableAttribute.DebuggingModes.Default`
flag to the generated Debuggable attribute
See https://msdn.microsoft.com/en-us/library/system.diagnostics.debuggableattribute.debuggingmodes(v=vs.110).aspx

Without this flag I see the following message when debugging unless I have "Debug project code only: do not step into framework code" unchecked to allow stepping into framework code.
```
Skipped loading symbols. Module is optimized and the debugger option 'Just My Code' is enabled.
```
Fixes https://bugzilla.xamarin.com/show_bug.cgi?id=55092
dsyme/FSharp.Compiler.Service,dsyme/FSharp.Compiler.Service,syeerzy/visualfsharp,Jand42/FSharp.Compiler.Service,dsyme/fsharp,dsyme/FSharp.Compiler.Service,sergey-tihon/FSharp.Compiler.Service,syeerzy/visualfsharp,dsyme/fsharp,cloudRoutine/FSharp.Compiler.Service,brettfo/visualfsharp,syeerzy/visualfsharp,dsyme/fsharp,cloudRoutine/FSharp.Compiler.Service,fsharp/fsharp,cloudRoutine/FSharp.Compiler.Service,Jand42/FSharp.Compiler.Service,fsharp/fsharp,dsyme/fsharp,dsyme/fsharp,dsyme/fsharp,syeerzy/visualfsharp,sergey-tihon/FSharp.Compiler.Service,brettfo/visualfsharp,ncave/FSharp.Compiler.Service,fsharp/fsharp,brettfo/visualfsharp,brettfo/visualfsharp,sergey-tihon/FSharp.Compiler.Service,syeerzy/visualfsharp,brettfo/visualfsharp,Jand42/FSharp.Compiler.Service,ncave/FSharp.Compiler.Service,ncave/FSharp.Compiler.Service,dsyme/fsharp,brettfo/visualfsharp,syeerzy/visualfsharp