Use default values instead of lambdas when possible. `std::nullopt` and
`nullptr` can be used now because of
https://github.com/llvm/llvm-project/pull/165724.

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.

Eliminate `llvm::` namespace qualifier for commonly used types in MLIR
TableGen backends to reduce code clutter.

This reverts commit 0eed3055511381436ee69d1caf64a4af47f8d65c and applies
additional fixes in `verifyArgument` in OmpOpGen.cpp for gcc-7 bot
failures

Reverts llvm/llvm-project#112316

Bots are failing.

Use const pointers for various `Init` objects. This is a part of effort
to have better const correctness in TableGen backends:


https://discourse.llvm.org/t/psa-planned-changes-to-tablegen-getallderiveddefinitions-api-potential-downstream-breakages/81089

Make sure that the usage of `cppType` and `cppClassName` of type and
attribute definitions/constraints is consistent in TableGen.

- `cppClassName`: The C++ class name of the type or attribute.
- `cppType`: The fully qualified C++ class name: C++ namespace and C++
class name.

Basically, we should always use the fully qualified C++ class name for
parameter types, return types or template arguments.

Also some minor cleanups.

Fixes #57279.

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