[clang] [compiler-rt] [compiler-rt:ubsan] Add -fsanitize=unaligned-pointer-subtraction to detect undefined subtraction of two pointers (PR #210635)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

@vitalybuka usually does those reviews.

https://github.com/llvm/llvm-project/pull/210635