[flang] Fix invalid iterator erasure in boxed-procedure pass (#79830)

The code in BoxedProcedureRewrite was erasing the mappings <old type,
new type> once "new type" was fully translated. This was done in an
invalid way because the map was an llvm::SmallMapVector that do not have
stable iterators, and new items were added to the map between the
creation of the iterator and its erasure.

It is anyway not needed and expensive to erase the types (see
llvm::MapVector note), the cache can be used for the whole pass, which
is very beneficial in the context of an apps using "big derived types"
(dozens of components/parents).