[flang] Retain internal and BIND(C) host procedure link in FIR (#87796)

Currently, it is not possible to find back which fun.func is the host
procedure of some internal procedure because the mangling of the
internal procedure does not contain info about the BIND(C) name of the
host.
This info may be useful to ensure dwarf DW_TAG_subprogram of internal
procedures are nested under DW_TAG_subprogram of host procedures for
instance.