[msan] Use real memset/memcpy/memmove in interceptors.

Our versions are not exactly as fast as libc's, and
MSan uses them heavily (even compared to other sanitizers).

This will break if libc version of mem* are instrumented,
but they never are, and if they are, we should be able
to fix it on libc side.



git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@212799 91177308-0d34-0410-b5e6-96231b3b80d8