[esan] Add __esan_report for mid-run data

Summary:
Adds a new public interface routine __esan_report() which can be used to
request profiling results prior to abnormal termination (e.g., for a server
process killed by its parent where the normal exit does not allow for
normal result reporting).

Implements this for the working-set tool.  The cache frag tool is left
unimplemented as it requires missing iteration capabilities.

Adds a new test.

Reviewers: aizatsky

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

Differential Revision: http://reviews.llvm.org/D22098

llvm-svn: 274964