Fix evaluation of `__has_include_next` during -frewrite-includes.

`__has_include_next` requires correct DirectoryLookup for being
evaluated correctly. We were using Preprocessor::GetCurDirLookup() but
we were calling it after the preprocessor finished its work. And in this
case CurDirLookup is always nullptr which makes `__has_include_next`
behave as `__has_include`.

Fix by storing and using CurDirLookup when preprocessor enters a file,
not when we rewrite the includes.

rdar://problem/36305026

Reviewers: bkramer

Reviewed By: bkramer

Subscribers: jkorous-apple, cfe-commits

Differential Revision: https://reviews.llvm.org/D45603

llvm-svn: 330041