clang-format: Add option to allow short blocks on a single line.

With AllowShortBlocksOnASingleLine, clang-format allows:
  if (a) { return; }

Based on patch by Gonzalo BG, thank you!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@208765 91177308-0d34-0410-b5e6-96231b3b80d8