Disallow array_ptrs to function types

[secure-sw-dev-bot]

This issue was copied from checkedc/checkedc#34

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Function types have no size associated with them. For this reason, it does not make sense to have array_ptrs to function types. Pointer arithmetic is not meaningful for these kinds of array_ptrs, which implies that we can't define bounds checks for these kinds of pointers.

[secure-sw-dev-bot]

Comment from @dtarditi:

The specification has been updated. Compiler support has been added for this check too.