[Demangle] Add prepend functionality to OutputString

Implement the functionallity of prepend, required by D demangler.
Please read discussion https://reviews.llvm.org/D111414 for context.
See also https://reviews.llvm.org/D111947 .

Reviewed By: dblaikie, Geod24

Differential Revision: https://reviews.llvm.org/D111948

GitOrigin-RevId: 2d77b272a8f9b5b89b022628ca30b6b896a8f725
1 file changed