[msan] Refactor memory layout specification and setup.

A flexible way of describing MSan memory layout details on various
platforms. No significant functional changes, but the memory layout
description that you get at verbosity=1 looks slightly different.
This change includes stronger sanity checks than before.

The goal of this change is to allow more than 2 application memory
ranges for https://code.google.com/p/memory-sanitizer/issues/detail?id=76.

llvm-svn: 227192