[InstCombine] Annotate strndup calls with dereferenceable_or_null

[InstCombine] Annotate strndup calls with dereferenceable_or_null

"Implementations are free to malloc() a buffer containing either (size + 1) bytes or (strnlen(s, size) + 1) bytes. Applications should not assume that strndup() will allocate (size + 1) bytes when strlen(s) is smaller than size."

llvm-svn: 372647