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.
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
