[LangRef] Clarify the behavior of memory access instructions when...

[LangRef] Clarify the behavior of memory access instructions when pointers/sizes aren't well-defined

This is a patch to LangRef that clarifies the behavior of load/store/memset/memcpy/memmove when the pointers or sizes are not well-defined
as well.

MSan detects a case when e.g., only lower bits of address are garbage when `-msan-check-access-address` is enabled, and it does not directly conflict with this patch because a C program should not use a pointer with undef bits and reasonable optimizations do not convert a well-defined pointer into a pointer with undef bits.

This patch contains a definition of a well-defined value as well.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D87994