Change clang-format's affinity for breaking after return types.

Function declarations are now broken with the following preferences:
1) break amongst arguments.
2) break after return type.
3) break after (.
4) break before after nested name specifiers.

Options #2 or #3 are preferred over #1 only if a substantial number of
lines can be saved by that.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179287 91177308-0d34-0410-b5e6-96231b3b80d8