[lldb] [llgs] Support owning and detaching extra processes

Add a NativeDelegate API to pass new processes (forks) to LLGS,
and support detaching them via the 'D' packet.  A 'D' packet without
a specific PID detaches all processes, otherwise it detaches either
the specified subprocess or the main process, depending on the passed
PID.

Differential Revision: https://reviews.llvm.org/D100191

lldb/source/Utility/StringExtractorGDBRemote.cpp

@@ -378,9 +378,7 @@ StringExtractorGDBRemote::GetServerPacketType() const {
     return eServerPacketType_C;
 
   case 'D':
-    if (packet_size == 1)
-      return eServerPacketType_D;
-    break;
+    return eServerPacketType_D;
 
   case 'g':
     return eServerPacketType_g;