Better support for constructor initializers.

We used to format initializers like this (with a sort of hacky implementation):
Constructor()
    : Val1(A),
      Val2(B) {

and now format like this (with a somewhat better solution):
Constructor()
    : Val1(A), Val2(B) {

assuming this would not fit on a single line. Also added tests.

As a side effect we now first analyze whether an UnwrappedLine needs to be
split at all. If not, not splitting it is the best solution by definition. As
this should be a very common case in normal code, not exploring the entire
solution space can provide significant speedup.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@170457 91177308-0d34-0410-b5e6-96231b3b80d8