TODO -> FIXME (#1316)

This is for https://google.github.io/styleguide/cppguide.html#TODO_Comments; FIXME seems to be getting used for the same purpose, and we should probably only have one so that it's easier to scan for.

explorer/ast/expression.cpp

@@ -238,7 +238,7 @@ void Expression::PrintID(llvm::raw_ostream& out) const {
       out << "Continuation";
       break;
     case ExpressionKind::ValueLiteral:
-      // FIXME: For layering reasons, we can't print out the value from here.
+      // TODO: For layering reasons, we can't print out the value from here.
       out << "ValueLiteral";
       break;
     case ExpressionKind::IndexExpression: