Identified with misc-include-cleaner.

This patch provides more information to the
`PPCallbacks::InclusionDirective()` hook. We now always pass the
suggested module, regardless of whether it was actually imported or not.
The extra `bool ModuleImported` parameter then denotes whether the
header `#include` will be automatically translated into import the the
module.

The main change is in `clang/lib/Lex/PPDirectives.cpp`, where we take
care to not modify `SuggestedModule` after it's been populated by
`LookupHeaderIncludeOrImport()`. We now exclusively use the `SM`
(`ModuleToImport`) variable instead, which has been equivalent to
`SuggestedModule` until now. This allows us to use the original
non-modified `SuggestedModule` for the callback itself.

(This patch turns out to be necessary for
https://github.com/apple/llvm-project/pull/8011).

This patch removes several "using" declarations and #include
"llvm/ADT/Optional.h".

This is part of an effort to migrate from llvm::Optional to
std::optional:

https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716

This patch replaces (llvm::|)Optional< with std::optional<.  I'll post
a separate patch to remove #include "llvm/ADT/Optional.h".

This is part of an effort to migrate from llvm::Optional to
std::optional:

https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716

This patch adds #include <optional> to those files containing
llvm::Optional<...> or Optional<...>.

I'll post a separate patch to actually replace llvm::Optional with
std::optional.

This is part of an effort to migrate from llvm::Optional to
std::optional:

https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716

The needed tweaks are mostly trivial, the one nasty bit is Clang's usage
of OptionalStorage. To keep this working old Optional stays around as
clang::CustomizableOptional, with the default Storage removed.
Optional<File/DirectoryEntryRef> is replaced with a typedef.

I tested this with GCC 7.5, the oldest supported GCC I had around.

Differential Revision: https://reviews.llvm.org/D140332

This reverts commit 8f0df9f3bbc6d7f3d5cbfd955c5ee4404c53a75d.

The Optional*RefDegradesTo*EntryPtr types want to keep the same size as
the underlying type, which std::optional doesn't guarantee. For use with
llvm::Optional, they define their own storage class, and there is no way
to do that in std::optional.

On top of that, that commit broke builds with older GCCs, where
std::optional was not trivially copyable (static_assert in the clang
sources was failing).

std::optional::value() has undesired exception checking semantics and is
unavailable in older Xcode (see _LIBCPP_AVAILABILITY_BAD_OPTIONAL_ACCESS). The
call sites block std::optional migration.

This makes `ninja clang` work in the absence of llvm::Optional::value.