Identified with llvm-use-ranges.

Identified with modernize-use-using.

Implement support for `subcommands` in OptTable to attain feature parity
with `cl`.

Design overview:
https://discourse.llvm.org/t/subcommand-feature-support-in-llvm-opttable/88098

Issue: https://github.com/llvm/llvm-project/issues/108307

Make `AppendZero` a class member instead of an argument to
`GetOrAddStringOffset` to reflect the intended usage that for a given
`StringToOffsetTable`, all strings must use the same value of
`AppendZero`.

Modify `EmitStringTableDef` to drop the `Indent` argument as its always
set to `""`, and to fail if it's called for a table with
non-null-terminated strings.

Rename `ListInit::getValues()` to `getElements()` to better match with
other `ListInit` members like `getElement`. Keep `getValues()` for
existing downstream code but mark it deprecated.

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

DenseSet, SmallPtrSet, SmallSet, SetVector, and StringSet recently
gained C++23-style insert_range.  This patch replaces:

  Dest.insert(Src.begin(), Src.end());

with:

  Dest.insert_range(Src);

This patch does not touch custom begin like succ_begin for now.