Ensure that DIType is regenerated after we visit an implementation
that adds ivars to an interface.

Fixes rdar://13175234

This is an update to r176116 that performs a smart caching of interfaces.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176584 91177308-0d34-0410-b5e6-96231b3b80d8