[llvm] Add a super simple wrapper for a merged string table. (PR #119488)

[Chandler Carruth via llvm-commits]

chandlerc wrote:

Note that this is intended for use cases like https://github.com/llvm/llvm-project/pull/119198 and some of the other string tables indexed by offset.

https://github.com/llvm/llvm-project/pull/119488