Reapply:

Author: Eric Christopher <echristo@apple.com>
Date:   Thu Aug 16 23:50:46 2012 +0000

    Add some caching here for the builtin types.

    rdar://12117935

    git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@162066 91177308-0d34-0410-b5e6-96231b3b80d8

after fixing a thinko.

llvm-svn: 162243