Only set the attached flag when a client attaches (#3490)

In defer mode, the debugger thread is started as soon as possible before
any clients are attached, but the debugger is_attached flag was being set
immediately. This would cause the runtime (and the System.Diagnostics.Debugger
class) to wrongly report a client as being attached.

Originally fixed in the Unity fork here: https://github.com/Unity-Technologies/mono/commit/797a210e0f2416d779c448bb9e03bb16933e261f