Committed the attached patch as r14-2582-g89d0f082b3c95f.

This is about OpenMP's uses_allocators clause to the 'target' directive.

Using the clause with predefined allocators as list arguments is
required if those allocators are used in a target region - unless
there is an 'omp requires dynamic_allocators' in the compilation unit.

While the later is a no op (requirement fulfilled by all devices), we
still had to handle the no op when using 'uses_allocators', which this
commit does.

However, uses_allocators also permits to define new allocators; for
those, this commit stops after parsing and resolving with a
'sorry, unimplemented'.

Support for the latter will be added together with the C/C++ support
by a re-diffed/updated version of Chung-Lin's patch at
https://gcc.gnu.org/pipermail/gcc-patches/2022-June/596587.html

(See thread for pending review issues; the C++ member var issue
is https://gcc.gnu.org/PR110347 )

Tobias
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