Fix clang-format bug in unwrapped-line merging.

Before (in styles that allow it), clang-format would not merge an
if statement onto a single line, if only the second line was format
(e.g. in an editor integration):

if (a)
  return; // clang-format invoked on this line.

With this patch, this gets properly merged to:

if (a) return; // ...

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181770 91177308-0d34-0410-b5e6-96231b3b80d8