This patch moves some generic code for Fortran out of gimplify.cc
to trans-openmp.cc and fixes several issues related to mapping.

Tested with nvptx offloading.
OK for mainline?


Caveats:

Besides the issues shown in the comment-out code, there remains also an
issue with implicit mapping - at least for deferred-length strings,
but I wouldn't be surprised if - at least depending on the used
'defaultmap' value (e.g. 'alloc') - there are also issues with array
descriptors.

Note:

Regarding the declare target check for mapping: Without declare
target, my assumption is that the hidden length variable will
get implicitly mapped if needed. Independent of deferred-length
or not, there is probably an issue with 'defaultmap(none)' and
the hidden variable. - In any case, I prefer to defer all those
issues to later (by having them captured in one/several PR).


Tobias

PS: This patch is a follow up to
   [Patch] Fortran/OpenMP: Fix DT struct-component with 'alloc' and array descr
   https://gcc.gnu.org/pipermail/gcc-patches/2022-November/604887.html
which fixed part of the problems. But as discussed on IRC, it did treat 'alloc'
as special and missed some other map types. - In addition, this patch has a
much extended test coverage and fixes some more issues found that way.
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