Don't simply give up when exceeding 80cols, choose an "ok" option.

This addresses llvm.org/PR14847.

We can now format something like:
int aaaaaaaaaaaaaaaaaaaaaaaaaaa =
    // aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaiaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
    + bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb;

clang-format unavoidably exceeds the column limit, but does not just
flush everything into a single line. Moreover, it tries to minimize the
number of characters beyond the column limit.