[MemoryBuiltins] Add field for alignment argument [NFC]

There are a few places where the alignment argument for AlignedAllocLike functions was previously hardcoded. This patch adds an getAllocAlignment function and a change to the MemoryBuiltin table to allow alignment arguments to be found generically.

This will shortly allow alignment inference on operator new's with align_val params and an extension to Attributor's HeapToStack.  The former will follow shortly - I split Bryce's patch for purpose of having the large change be NFC.  The later will be reviewed separately.

Differential Revision: https://reviews.llvm.org/D116851 (part 1 of 2)