This patch adds parsing/argument-checking support for
   '!$omp allocators allocate([align(int),allocator(a) :] list)'

This is kind of logical follow-up and prep patch for the
   '!$omp allocate(list) [align(v) allocator(a)]'
support that was submitted as part of a larger patchset by Abid; cf.
review at
   "[PATCH 1/5] [gfortran] Add parsing support for allocate directive (OpenMP 5.0)."
   https://gcc.gnu.org/pipermail/gcc-patches/2022-October/603258.html

My follow-up patch will add parsing support for declarative/executable '!$omp allocate'.

OK for mainline?

Tobias
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