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/libmain/shared.cc

@@ -23,7 +23,9 @@ namespace nix {
 static bool gcWarning = true;
 
 void printGCWarning() {
-  if (!gcWarning) return;
+  if (!gcWarning) {
+    return;
+  }
 
   static bool haveWarned = false;
   if (!haveWarned) {