[patch] libstdc++/70940 Start fixing polymorphic memory resources

[Jonathan Wakely]

This fixes the first errors noted in the PR, so that resource_adaptor
doesn't rely on anything that isn't guaranteed by the Allocator
requirements, and __null_memory_resource::do_is_equal doesn't have a
missing return.

More work is needed to solve the alignment bugs, which I'll work on
soon.

Tested x86_64-linux, committed to trunk.

I'll backport these fixes to gcc-6 as well once they're complete.


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