clang-format: Leave empty lines within UnwrappedLines.

These are commonly used to structure things like enums or long braced
lists. There doesn't seem to be a good reason to have the behavior in
such structures be different from the behavior between statements.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210183 91177308-0d34-0410-b5e6-96231b3b80d8