clang-format: Fix bad indentation of nested blocks.

Before:
  DEBUG(  //
  { f(); });

After:
  DEBUG(  //
      { f(); });

Also add additional test to selected formatting of individual statements
in nested blocks.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195952 91177308-0d34-0410-b5e6-96231b3b80d8