Identified with llvm-use-ranges.

Use `DefaultUnreachable` from
https://github.com/llvm/llvm-project/pull/161970.

Add `llvm::uninitialized_copy` that accepts a range instead of start/end
iterator for the source of the copy.

I observed that we have the boundary comments in the codebase like:

```
//===----------------------------------------------------------------------===//
// ...
//===----------------------------------------------------------------------===//
```

I also observed that there are incomplete boundary comments. The
revision is generated by a script that completes the boundary comments.

```
//===----------------------------------------------------------------------===//
// ...

...
```

Signed-off-by: hanhanW <hanhan0912@gmail.com>

This commit enables the expression of negated native constraints in PDLL:

If a constraint is prefixed with "not" it is parsed as a negated constraint and hence the attribute `isNegated` of the emitted `pdl.apply_native_constraint` operation is set to `true`.
In first instance this is only supported for the calling of external native C++ constraints and generation of PDL patterns.

Previously, negating a native constraint would have  been handled by creating an additional native call, e.g.
```PDLL
Constraint checkA(input: Attr);
Constarint checkNotA(input: Attr);
```
or by including an explicit additional operand for negation, e.g.
`Constraint checkA(input: Attr, negated: Attr);`

With this a constraint can simply be negated by prefixing it with `not`. e.g.
```PDLL
Constraint simpleConstraint(op: Op);

Pattern example {
    let inputOp = op<test.bar>() ->(type: Type);
    let root = op<test.foo>(inputOp.0) -> ();
    not simpleConstraint(inputOp);
    simpleConstraint(root);
    erase root;
}
```

Depends on [[ https://reviews.llvm.org/D153871 | D153871 ]]

Reviewed By: Mogball

Differential Revision: https://reviews.llvm.org/D153959

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

This allows for constructing type and value ranges from various sub elements,
which makes it easier to construct operations that take a range as an operand
or result type. Range construction is currently limited to within rewrites, to match
the current constraint on the PDL side.

Differential Revision: https://reviews.llvm.org/D133720

This commit enables providing long-form documentation more seamlessly to the LSP
by revamping decl documentation. For ODS imported constructs, we now also import
descriptions and attach them to decls when possible. For PDLL constructs, the LSP will
now try to provide documentation by parsing the comments directly above the decls
location within the source file. This commit also adds a new parser flag
`enableDocumentation` that gates the import and attachment of ODS documentation,
which is unnecessary in the normal build process (i.e. it should only be used/consumed
by tools).

Differential Revision: https://reviews.llvm.org/D124881