Improve formatting of conditional expressions.

Before we did not really systematically format those. Now, we format the
different cases as:

- 1 Line:  a ? b : c;
- 2 Lines: short ? loooooooooong
                 : loooooooooong
- 2 Lines: loooooooooooooooong
               ? short : short
- 3 Lines: loooooooooooooooong
               ? loooooooooooooong
               : loooooooooooooong

Not sure whether "?" and ":" should go on the new line, but it seems to
be the most consistent approach.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173683 91177308-0d34-0410-b5e6-96231b3b80d8