[lldb] [gdb-server] Fix fill_clamp to handle signed src types

[mgorny]

Summary

Fix the fill_clamp() function to handle signed source types. Make sure
that the source value is always non-negative, and cast it to unsigned
when verifying the upper bound. This fixes compiler warnings about
comparing unsigned and signed types.

[labath]

lldb/source/Plugins/Process/gdb-remote/GDBRemoteCommunicationServerCommon.cpp

776

T will always be unsigned here, right? May not hurt to add a static_assert for that, mainly to avoid people wondering if this logic is correct...

[mgorny]

lldb/source/Plugins/Process/gdb-remote/GDBRemoteCommunicationServerCommon.cpp

776

Yes, and that makes the logic much simpler. I'll add the assert. Thanks!

lldb/source/Plugins/Process/gdb-remote/GDBRemoteCommunicationServerCommon.cpp

if (packet.GetChar() == ',') {
       return SendPacketNoLock(response.GetString());
     }
   }
   return SendErrorResponse(24);
 }
 
 template <typename T, typename U>
 static void fill_clamp(T &dest, U src, typename T::value_type fallback) {
-  dest = src <= std::numeric_limits<typename T::value_type>::max() ? src
-                                                                   : fallback;
+  static_assert(std::is_unsigned<typename T::value_type>::value,
+                "Destination type must be unsigned.");
+  using UU = typename std::make_unsigned<U>::type;
+  constexpr auto T_max = std::numeric_limits<typename T::value_type>::max();
+  dest = src >= 0 && static_cast<UU>(src) <= T_max ? src : fallback;
 }
 
 GDBRemoteCommunication::PacketResult
 GDBRemoteCommunicationServerCommon::Handle_vFile_FStat(
     StringExtractorGDBRemote &packet) {
   StreamGDBRemote response;
   packet.SetFilePos(::strlen("vFile:fstat:"));
   int fd = packet.GetS32(-1, 16);