- Also eliminate unneeded std::string() around some literal strings.

Also use brace initialization and emplace to avoid explicitly 
constructing std::pair, and the same for std::tuple.

Some clients do not want to emit a terminator after each sub-sequence
(they have other means of determining the length of sub-sequences).

This moves `Term` argument from `emit` method to the constructor and
makes it optional. It couldn't be made optional while still on the
`emit` method because if the terminator wasn't specified, it has to be
taken into account in `layout` method as well.

The fact that `layout` method was called is now recorded in a dedicated
member variable, `IsLaidOut`. `Entries != 0` can no longer be used to
reliably check if `layout` method was called because it may be zero for
a different reason: the terminator wasn't specified and all added
sequences (if any) were empty.

This reduces the size of `*LaneMaskLists` and `*SubRegIdxLists` a bit
and resolves the removed TODO.

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

Change DFAEmitter 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

Migrate CTags/DFA/Directive Emitters to const RecordKeeper.

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

These are unnecessary since C++17.