[esan|wset] Iterate all memory to compute the total working set

Summary:
Adds iteration of all application memory in an efficient manner using
shadow faults.  Shadow memory starts out inaccessible and we mark it
writable one page at a time on each fault when the instrumentation touches
it.  This allows iteration over just the mapped shadow memory, saving
significant time.

Adds a process-end iteration and pretty-printing of the final result.

Adds a new test and updates the existing tests.

Reviewers: aizatsky, filcab

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

Differential Revision: http://reviews.llvm.org/D20578

llvm-svn: 271277