Add working set base runtime library

Summary:
Adds the base runtime library for the working set tool.
Adds slowpath code for updating the shadow memory.

To be added in the future:
+ Scan memory and report the total size.
+ Take samples for intermediate values.

Reviewers: aizatsky

Subscribers: kubabrecka, vitalybuka, zhaoqin, kcc, eugenis, llvm-commits

Differential Revision: http://reviews.llvm.org/D20485

llvm-svn: 270650