clang-format: Fix corner cases for comments in if conditions.

Before:
  if ( // a
          x + 3) { ..

After:
  if ( // a
      x + 3) { ..

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@208175 91177308-0d34-0410-b5e6-96231b3b80d8