A first step towards giving format tokens pointer identity.

With this patch, we create all tokens in one go before parsing and pass
an ArrayRef<FormatToken*> to the UnwrappedLineParser. The
UnwrappedLineParser is switched to use pointer-to-token internally.

The UnwrappedLineParser still copies the tokens into the UnwrappedLines.
This will be fixed in an upcoming patch.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@182768 91177308-0d34-0410-b5e6-96231b3b80d8