clang-tidy can make syntactic changes to your code using FIXITS
clang-tidy YOUR_FILE.cpp -fix -checks="readability-braces-around-statements" -- COMPILE_OPTIONS
Updated:
clang-tidy is a bit of a heavyweight tool for this as it needs compile options to parse the file, sadly clang-format (as of v3.9) won’t add braces.
COMPILE_OPTIONS would be the include paths etc that you use to compile the file with, ie -std=c++14 -stdlib=libc++ -O2 -I.
If you have a compile_options.json file from CMake then you can pass the path of the directory it is contained in to clang-tidy and it will look up the appropriate compile options for the file:
clang-tidy YOUR_FILE.cpp -fix -checks="readability-braces-around-statements" -p COMPILE_OPTIONS_DIR