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

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.

Differential Revision: https://reviews.llvm.org/D113519

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

@@ -771,8 +771,11 @@ GDBRemoteCommunicationServerCommon::Handle_qPlatform_shell(
 
 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