libgo: update to final Go 1.18 release

gcc/go/gofrontend/MERGE

libgo/MERGE

libgo/VERSION

libgo/go/cmd/go/alldocs.go

libgo/go/cmd/go/internal/modfetch/coderepo.go

libgo/go/cmd/go/internal/modfetch/coderepo_test.go

libgo/go/cmd/go/internal/modload/init.go

libgo/go/cmd/go/internal/modload/modfile.go

libgo/go/cmd/go/internal/run/run.go

libgo/go/cmd/go/internal/vcs/vcs.go

libgo/go/cmd/go/internal/workcmd/edit.go

libgo/go/cmd/go/internal/workcmd/init.go

libgo/go/cmd/go/internal/workcmd/sync.go

libgo/go/cmd/go/internal/workcmd/use.go

libgo/go/cmd/go/internal/workcmd/work.go

libgo/go/cmd/go/testdata/script/run_work_versioned.txt

...o/cmd/go/testdata/script/test_fuzz_minimize_dirty_cov.txt

...cmd/go/testdata/script/test_fuzz_minimize_interesting.txt

libgo/go/cmd/go/testdata/script/version_buildvcs_git.txt

libgo/go/cmd/go/testdata/script/work.txt