clang-format: Store pointers to seen formatting states.

As the memory ownership is handled by the SpecificBumpPtrAllocator
anyway, there is no need to duplicate states when inserting them into
the Seen-set. This leads to an improvement of ~10% on the benchmark
formatting file.

No functional changes intended.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209422 91177308-0d34-0410-b5e6-96231b3b80d8