Add msan custom mapping options.

Similarly to https://reviews.llvm.org/D18865 this adds options to provide custom mapping for msan.
As discussed in http://lists.llvm.org/pipermail/llvm-dev/2018-February/121339.html

Patch by vit9696(at)avp.su.

Differential Revision: https://reviews.llvm.org/D44926

llvm-svn: 328830