Revert "Revert "Revert "[clang][pp] adds '#pragma include_instead'"""

> Includes regression test for problem noted by @hans.
> is reverts commit 973de718.
>
> Differential Revision: https://reviews.llvm.org/D106898

Feature implemented as-is is fairly expensive and hasn't been used by
libc++. A potential reimplementation is possible if libc++ become
interested in this feature again.

Differential Revision: https://reviews.llvm.org/D123885