unique_ptrify some parameters to PTHManager::PTHManager

A couple of these arguments were passed by void* as a rather extreme
example of pimpling. Adjusting this to a more classic form of the idiom
(involving forward declarations) makes this more legible and allows
explicit passing of ownership via std::unique_ptr.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@216785 91177308-0d34-0410-b5e6-96231b3b80d8