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/json.cc

@@ -102,7 +102,9 @@ void JSONWriter::comma() {
   } else {
     state->str << ',';
   }
-  if (state->indent) indent();
+  if (state->indent) {
+    indent();
+  }
 }
 
 void JSONWriter::indent() {
@@ -116,7 +118,9 @@ void JSONList::open() {
 
 JSONList::~JSONList() {
   state->depth--;
-  if (state->indent && !first) indent();
+  if (state->indent && !first) {
+    indent();
+  }
   state->str << "]";
 }
 
@@ -143,7 +147,9 @@ void JSONObject::open() {
 JSONObject::~JSONObject() {
   if (state) {
     state->depth--;
-    if (state->indent && !first) indent();
+    if (state->indent && !first) {
+      indent();
+    }
     state->str << "}";
   }
 }