[esan|wset] Fix flaky sampling tests

Adds a new esan public interface routine __esan_get_sample_count() and uses
it to ensure that tests of sampling receive the minimum number of samples.

llvm-svn: 275948
diff --git a/compiler-rt/lib/esan/working_set.h b/compiler-rt/lib/esan/working_set.h
index 38ff063..6a976c3 100644
--- a/compiler-rt/lib/esan/working_set.h
+++ b/compiler-rt/lib/esan/working_set.h
@@ -24,6 +24,7 @@
 void initializeShadowWorkingSet();
 int finalizeWorkingSet();
 void reportWorkingSet();
+unsigned int getSampleCountWorkingSet();
 void processRangeAccessWorkingSet(uptr PC, uptr Addr, SIZE_T Size,
                                   bool IsWrite);