Tested a theory on the where when we lookup things in the accelerator tables

that if we prefer the current compile unit, followed by any compile units that
already had their DIEs parsed, followed by the rest of the matches, that we
might save some memory. This turned out not to help much. The code is commented
out, but I want to check it in so I don't lose the code in case it could help
later.

Added the ability to efficiently find the objective C class implementation
when using the new .apple_types acclerator tables with the type flags. If the
type flags are not available, we default back to what we were doing before.

llvm-svn: 146250