clang-format: Understand functions with decltype return type.

Before:
  decltype(long_name_forcing_break)
      f() {}

After:
  decltype(long_name_forcing_break)
  f() {}

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207965 91177308-0d34-0410-b5e6-96231b3b80d8