Identified with modernize-use-using.

`Predicates` and `Features` fields serve the same purpose. They should
be kept in sync, but not all predicates are based on features. This
resulted in introducing dummy features for that only reason.

This patch removes `Features` field and changes TableGen emitters to use
`Predicates` instead.

Historically, predicates were written with the assumption that the
checking code will be used in `SelectionDAGISel` subclasses, meaning
they will have access to the subclass variables, such as `Subtarget`.
There are no such variables in the generated
`GenSubtargetInfo::getHwModeSet()`, so we need to provide them. This can
be achieved by subclassing `HwModePredicateProlog`, see an example in
`Hexagon.td`.

Migrate CodeGenHWModes to use const RecordKeeper and const Record
pointers.

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

- Fix include guards for headers under utils/TableGen to match their
paths.

Refactor of the llvm-tblgen source into:
- a "Basic" library, which contains the bare minimum utilities to build
`llvm-min-tablegen`
- a "Common" library which contains all of the helpers for TableGen
backends. Such helpers can be shared by more than one backend, and even
unit tested (e.g. CodeExpander is, maybe we can add more over time)

Fixes #80647