[lldb] Fix GetTerminalWidth after afd4690

afd4690 fixed the type of the term-width setting but the getter
(Debugger::GetTerminalWidth) was still trying to get the terminal width
as an unsigned. This fixes TestXMLRegisterFlags.py.

(cherry picked from commit 354401f)

Author: JDevlieghere

lldb/source/Core/Debugger.cpp

@@ -365,7 +365,7 @@ bool Debugger::SetREPLLanguage(lldb::LanguageType repl_lang) {
 
 uint64_t Debugger::GetTerminalWidth() const {
   const uint32_t idx = ePropertyTerminalWidth;
-  return GetPropertyAtIndexAs<int64_t>(
+  return GetPropertyAtIndexAs<uint64_t>(
       idx, g_debugger_properties[idx].default_uint_value);
 }