Flang diverges from the llvm coding style in that it requires braces
around the bodies of if/while/etc statements, even when the body is
a single statement.
This commit adds the readability-braces-around-statements check to
flang's clang-tidy config file. Hopefully the premerge bots will pick it
up and report violations in Phabricator.
We also explicitly disable the check in the directories corresponding to
the Lower and Optimizer libraries, which rely heavily on mlir and llvm
and therefore follow their coding style.