[flang] Remove unnecessary unique_ptr.

The unique_ptr around the vector of char was unnecessary. We can just
move the vector itself and it still keeps the same stream of chars.

Original-commit: flang-compiler/f18@24672adba9fb38703cc88b2b7ff35b6c8824e918
Reviewed-on: https://github.com/flang-compiler/f18/pull/149