[LLD] [MinGW] Hook up --icf=safe to -opt:safeicf (#70037)

Back when the --icf= option was hooked up in the MinGW frontend in LLD,
in 2017, lld-link didn't support safe ICF, and mapping it to noicf was
suggested in review: https://reviews.llvm.org/D40019

In 2018, in ab038025,
lld-link did get support for handling address significance tables,
allowing the ICF to operate safely on more sections.

Later in 2021, lld-link did get support for a separate safe ICF mode in
5bdc5e7e /
https://reviews.llvm.org/D97436.

Hook this up for the MinGW frontend as well.