Change SubtargetFeatureInfo to use 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

Split RecordKeeper `getAllDerivedDefinitions` family of functions into
two variants:
  (a) non-const ones that return vectors of `Record *` and 
  (b) const ones, that return vector/ArrayRef of `const Record *`.

This will help gradual migration of TableGen backends to use 
`const RecordKeeper` and by implication change code to work 
with const pointers and better const correctness.

Existing backends are not yet compatible with the const family of
functions, so change them to use a non-constant `RecordKeeper` 
reference, till they are migrated.

Coverity (a static analysis tool) reported that the emitted 'Features'
variable inside emitComputeAvailableFeatures in TableGen might be
unitialized.
Silence this warning by adding brackets for the default initialization.
Adapt test cases to take additional brackets into account.

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