[clang][lex] Remove `Preprocessor::GetCurDirLookup()`

`Preprocessor` exposes the search directory iterator via `GetCurDirLookup()` getter, which is only used in two static functions.

To simplify reasoning about search directory iterators/references and to simplify the `Preprocessor` API, this patch makes the two static functions private member functions and removes the getter entirely.

Depends D119708.

Reviewed By: ahoppen, dexonsmith

Differential Revision: https://reviews.llvm.org/D119714