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

> `#pragma clang include_instead(<header>)` is a pragma that can be used
> by system headers (and only system headers) to indicate to a tool that
> the file containing said pragma is an implementation-detail header and
> should not be directly included by user code.
>
> The library alternative is very messy code that can be seen in the first
> diff of D106124, and we'd rather avoid that with something more
> universal.
>
> This patch takes the first step by warning a user when they include a
> detail header in their code, and suggests alternative headers that the
> user should include instead. Future work will involve adding a fixit to
> automate the process, as well as cleaning up modules diagnostics to not
> suggest said detail headers. Other tools, such as clangd can also take
> advantage of this pragma to add the correct user headers.
>
> Differential Revision: https://reviews.llvm.org/D106394

This caused compiler crashes in Chromium builds involving PCH and an include
directive with macro expansion, when Token::getLiteralData() returned null. See
the code review for details.

This reverts commit e8a64e54.