required for correctness in cases of copying a struct to itself or to 
an overlapping struct (itself for cases like *a = *a, and overlapping 
is possible with unions).

Hopefully, this won't end up being a perf issue; LLVM *should* be able 
to optimize memmove to memcpy in a lot of cases, and for small copies 
the generated code *should* be mostly comparable. (In reality, LLVM 
is currently horrible at optimizing memmove, but that's a bug, not a 
fundamental issue.)

gcc currently generates wrong code; that's 
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32667.

llvm-svn: 51566