[Demangle] Rename OutputStream to OutputString

This patch is a refactor to implement prepend afterwards. Since this changes a lot of files and to conform with guidelines, I will separate this from the implementation of prepend. Related to the discussion in https://reviews.llvm.org/D111414 , so please read it for more context.

Reviewed By: #libc_abi, dblaikie, ldionne

Differential Revision: https://reviews.llvm.org/D111947

GitOrigin-RevId: 2e97236aacbbcd58873826deb9a0e6a0704be8db
2 files changed