Implement caching of copy assignment operator lookup.

I believe, upon, careful review, that this code causes us to incorrectly
handle exception specifications of copy assignment operators in C++03
mode. However, we currently do not seem to properly implement the subtle
distinction between copying of members and bases made by implicit copy
constructors and assignment operators in C++03 - namely that they are
limited in their overload selection - in all cases. As such, I feel that
committing this code is correct pending a careful review of our
implementation of these semantics.

llvm-svn: 132841