Make single-line if statements optional.

Now, "if (a) return;" is only allowed, if this option is set.

Also add a Chromium style which is currently identical to Google style
except for this option.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@172431 91177308-0d34-0410-b5e6-96231b3b80d8