libclang: refactor handling of unsaved_files

Consolidate CXUnsavedFile argument handling in API functions to support a wider
cleanup of various file remapping schemes in the frontend.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@212427 91177308-0d34-0410-b5e6-96231b3b80d8