After the original API change to DefaultTimingManager::setOutput() (see
362aa434cc31ccca96749a6db8cd97f5b7d71206), users are forced to provide
their own implementation of OutputStrategy. However, default MLIR
implementations are usually sufficient. Expose Text and Json strategies
via factory-like method to avoid the problem in downstream projects.

This patch introduces StringSetTag, a dedicated empty struct to serve
as the "value type" for llvm::StringSet.  This change is part of an
effort to reduce the use of std::nullopt_t outside the context of
std::optional.

This patch addresses two use-after-move issues:

1. `Timing.cpp` A variable was std::moved and then immediately passed to
an `assert()` check. Since the moved-from state made the assertion
condition trivially true, the check was effectively useless. The
`assert()` is removed.

2. `Query.cpp` The `matcher` object was moved-from and then subsequently
used as if it still retained valid state. The fix ensures no subsequent
use for the moved-from variable.

Testing:
`ninja check-mlir`

These are identified by misc-include-cleaner.  I've filtered out those
that break builds.  Also, I'm staying away from llvm-config.h,
config.h, and Compiler.h, which likely cause platform- or
compiler-specific build failures.

`DefaultTimingManager::clear()` uses `out` to initialize `TimerImpl`,
but the `out` is `nullptr` by default. This means if
`DefaultTimingManager::setOutput()` is never called,
`DefaultTimingManager` destructor may generate SIGSEGV.

Revise the printing functionality of TimingManager to accommodate
various output formats. At present, TimingManager is limited to
outputting data solely in plain text format. To overcome this
limitation, I have introduced an abstract class that serves as the
foundation for printing. This approach allows users to implement
additional output formats by extending this abstract class. As part of
this update, I have integrated support for JSON as a new output format,
enhancing the ease of parsing for subsequent processing scripts.

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 patch mechanically replaces None with std::nullopt where the
compiler would warn if None were deprecated.  The intent is to reduce
the amount of manual work required in migrating from Optional to
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