[lsan] Add leak_check_at_exit flag.

We needed a way to tell LSan to invoke leak checking only if __do_leak_check()
is called explicitly. This can now be achieved by setting
leak_check_at_exit=false.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@187578 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/lsan/lsan.cc b/lib/lsan/lsan.cc
index 79e593f..22e18e2 100644
--- a/lib/lsan/lsan.cc
+++ b/lib/lsan/lsan.cc
@@ -31,6 +31,7 @@
   cf->fast_unwind_on_malloc = true;
   cf->malloc_context_size = 30;
   cf->detect_leaks = true;
+  cf->leak_check_at_exit = true;
 
   ParseCommonFlagsFromString(GetEnv("LSAN_OPTIONS"));
 }
@@ -59,7 +60,8 @@
   }
 
   InitCommonLsan();
-  Atexit(DoLeakCheck);
+  if (common_flags()->detect_leaks && common_flags()->leak_check_at_exit)
+    Atexit(DoLeakCheck);
 }
 
 }  // namespace __lsan