libclang: some type safety for CXTranslationUnitImpl's internals

There is no reason to store pointers as 'void*' in CXTranslationUnitImpl, since
it does not affect libclang ABI.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173588 91177308-0d34-0410-b5e6-96231b3b80d8