[LangRef] Adjust guarantee for llvm.memcpy to also allow equal arguments.

This adjusts the description of `llvm.memcpy` to also allow operands
to be equal. This is in line with what Clang currently expects.

This change is intended to be temporary and followed by re-introduce
a variant with the non-overlapping guarantee for cases where we can
actually ensure that property in the front-end.

See the links below for more details:
http://lists.llvm.org/pipermail/cfe-dev/2020-August/066614.html
and PR11763.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D86815