clang-format: Prefer breaking before trailing return arrows.

Before:
  auto SomeFunction(
      A aaaaaaaaaaaaaaaaaaaaa) const -> decltype(f(aaaaaaaaaaaaaaaaaaaaa)) {}

After:
  auto SomeFunction(A aaaaaaaaaaaaaaaaaaaaa) const
      -> decltype(f(aaaaaaaaaaaaaaaaaaaaa)) {}

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@220043 91177308-0d34-0410-b5e6-96231b3b80d8