[Alignment][NFC] TargetLowering::allowsMisalignedMemoryAccesses

Summary:
Note to downstream target maintainers: this might silently change the semantics of your code if you override `TargetLowering::allowsMisalignedMemoryAccesses` without marking it override.

This patch is part of a series to introduce an Alignment type.
See this thread for context: http://lists.llvm.org/pipermail/llvm-dev/2019-July/133851.html
See this patch for the introduction of the type: https://reviews.llvm.org/D64790

Reviewers: courbet

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81374