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/libexpr/value-to-json.cc

@@ -13,7 +13,9 @@ void printValueAsJSON(EvalState& state, bool strict, Value& v,
                       JSONPlaceholder& out, PathSet& context) {
   checkInterrupt();
 
-  if (strict) state.forceValue(v);
+  if (strict) {
+    state.forceValue(v);
+  }
 
   switch (v.type) {
     case tInt: