Change the data structure used in clang-format.

This is a first step towards supporting more complex structures such
as #ifs inside unwrapped lines. This patch mostly converts the array-based
UnwrappedLine into a linked-list-based UnwrappedLine. Future changes will
allow multiple children for each Token turning the UnwrappedLine into a
tree.

No functional changes intended.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@171856 91177308-0d34-0410-b5e6-96231b3b80d8