[flang][openmp] fix OMPFunctionFiltering pass after #87796 (#89776)

The pass assumed that all fun.func symbol usages could be safely
replaced by undef, that is not true after #87796 that added a back link
from internal procedure back to the parent procedure. This caused the
internal procedures to be erased and then processed (segfault).

Also set visibility of such internal procedures so that MLIR do not remove them before
the target function is generated for the target region.