[OPENMP]Allow no allocator clause in target regions with requires

[OPENMP]Allow no allocator clause in target regions with requires
dynamic_allocators.

According to the OpenMP 5.0, 2.11.3 allocate Directive, Restrictions,
allocate directives that appear in a target region must specify an
allocator clause unless a requires directive with the dynamic_allocators
clause is present in the same compilation unit. Patch adds a check for a
presence of the requires directive with the dynamic_allocators clause.

llvm-svn: 356758