Extend runtime bounds checks to handle relative alignment.

[secure-sw-dev-bot]

This issue was copied from checkedc/checkedc-clang#329

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If the relative alignment of a pointer p with array_ptr is not an integral multiple of sizeof(T), we need to adjust the bounds check slightly (as described in the Checked C specification).

[secure-sw-dev-bot]

Comment from @sulekhark:

#937 contains an example where the runtime bounds check needs to be adjusted to handle relative alignment.