[flang] (Re-)Enable alias tags pass by default (#74250)

Enable by default for optimization levels higher than 0 (same behavior
as clang).

For simplicity, only forward the flag to the frontend driver when it
contradicts what is implied by the optimization level.

This was first landed in
https://github.com/llvm/llvm-project/pull/73111 but was later reverted
due to a performance regression. That regression was fixed by
https://github.com/llvm/llvm-project/pull/74065.