Update year in license files

In last year's update (D48219) it was suggested that the release manager
might want to do this, so here we go.

git-svn-id: https://llvm.org/svn/llvm-project/libclc/trunk@351194 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/LICENSE.TXT b/LICENSE.TXT
index 6453fb6..15448ca 100644
--- a/LICENSE.TXT
+++ b/LICENSE.TXT
@@ -11,7 +11,7 @@
 
 ==============================================================================
 
-Copyright (c) 2011-2018 by the contributors listed in CREDITS.TXT
+Copyright (c) 2011-2019 by the contributors listed in CREDITS.TXT
 
 All rights reserved.