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/nixexpr.hh

@@ -23,13 +23,25 @@ MakeError(EvalError, Error) MakeError(ParseError, Error)
       : file(file), line(line), column(column){};
   operator bool() const { return line != 0; }
   bool operator<(const Pos& p2) const {
-    if (!line) return p2.line;
-    if (!p2.line) return false;
+    if (!line) {
+      return p2.line;
+    }
+    if (!p2.line) {
+      return false;
+    }
     int d = ((string)file).compare((string)p2.file);
-    if (d < 0) return true;
-    if (d > 0) return false;
-    if (line < p2.line) return true;
-    if (line > p2.line) return false;
+    if (d < 0) {
+      return true;
+    }
+    if (d > 0) {
+      return false;
+    }
+    if (line < p2.line) {
+      return true;
+    }
+    if (line > p2.line) {
+      return false;
+    }
     return column < p2.column;
   }
 };