[DebugInfo] Replace unreachable with None

Invalid user input should not trigger assertions and unreachables. We
already return an Option so we should just return None here.

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=5532

llvm-svn: 327274

lib/DebugInfo/DWARF/DWARFFormValue.cpp

@@ -161,7 +161,7 @@ DWARFFormValue::getFixedByteSize(dwarf::Form Form,
     return 0;
 
   default:
-    llvm_unreachable("Handle this form in this switch statement");
+    break;
   }
   return None;
 }