[llvm-objdump] Add llvm_unreachable to silence GCC warning. NFC.

GCC 7 warned about control reaching the end of the non-void function,
despite all 7 LineChar values being handled in the switch.

llvm/tools/llvm-objdump/llvm-objdump.cpp

@@ -798,6 +798,7 @@ class LiveVariablePrinter {
     case LineChar::LabelHoriz:
       return IsASCII ? "-" : "─";
     }
+    llvm_unreachable("Unexpected LineChar");
   }
 
   /// Print live ranges to the right of an existing line. This assumes the