Remove unnecessary call of DenseMap::clear.

It should be always true that _liveAtoms is empty, so we don't have to clear it.
Add an assert() instead.

llvm-svn: 198040
diff --git a/lld/lib/Core/Resolver.cpp b/lld/lib/Core/Resolver.cpp
index d10eaf2..85026aa 100644
--- a/lld/lib/Core/Resolver.cpp
+++ b/lld/lib/Core/Resolver.cpp
@@ -328,9 +328,7 @@
   // only do this optimization with -dead_strip
   if (!_context.deadStrip())
     return;
-
-  // clear liveness on all atoms
-  _liveAtoms.clear();
+  assert(_liveAtoms.empty());
 
   // By default, shared libraries are built with all globals as dead strip roots
   if (_context.globalsAreDeadStripRoots()) {