Switch NULL to C++11 nullptr in source/Symbol and source/Utility

Patch by Robert Matusewicz

llvm-svn: 206713
diff --git a/lldb/source/Symbol/ClangExternalASTSourceCommon.cpp b/lldb/source/Symbol/ClangExternalASTSourceCommon.cpp
index 697dc7e..650d252 100644
--- a/lldb/source/Symbol/ClangExternalASTSourceCommon.cpp
+++ b/lldb/source/Symbol/ClangExternalASTSourceCommon.cpp
@@ -36,7 +36,7 @@
     if (HasMetadata (object))
         return &m_metadata[object];
     else
-        return NULL;
+        return nullptr;
 }
 
 void