Avoid naming variable after type to fix GCC 5.3 build

GCC says:
.../llvm/lib/DebugInfo/GSYM/FunctionInfo.cpp:195:12:
error: ‘InfoType’ is not a class, namespace, or enumeration
       case InfoType::EndOfList:
                   ^

Presumably, GCC thinks InfoType is a variable here. Work around it by
using the name IT as is done above.