style(3p/nix): Enforce braces around loops and conditionals

This change was generated with:

  fd -e cc -e hh | xargs -I{} clang-tidy {} -p ~/projects/nix-build/ \
    --checks='-*,readability-braces-around-statements' --fix \
    -fix-errors

Some manual fixes were applied because some convoluted unbraced
statements couldn't be untangled by clang-tidy.

This commit still includes invalid files, but I decided to clean them
up in a subsequent commit so that it becomes more obvious where
clang-tidy failed. Maybe this will allow for a bug-report to
clang-tidy.

third_party/nix/src/libutil/xml-writer.cc

@@ -13,13 +13,17 @@ XMLWriter::XMLWriter(bool indent, std::ostream& output)
 XMLWriter::~XMLWriter() { close(); }
 
 void XMLWriter::close() {
-  if (closed) return;
+  if (closed) {
+    return;
+  }
   while (!pendingElems.empty()) closeElement();
   closed = true;
 }
 
 void XMLWriter::indent_(size_t depth) {
-  if (!indent) return;
+  if (!indent) {
+    return;
+  }
   output << string(depth * 2, ' ');
 }