Fix PR4922, where Sema would complete tentative definitions in nondeterminstic

order because it was doing so while iterating over a densemap.

There are still similar problems in other places, for example 
WeakUndeclaredIdentifiers is still written to the PCH file in a nondeterminstic
order, and we emit warnings about #pragma weak in nondeterminstic order.

llvm-svn: 81236