Remove the only use of LookupResult's implicit copy ctor

LookupResult should not be copyable, it's not readily copyable and can
only be copied when it's in specific states (in a query state, without
any results, basically). Instead, just extract the /query/ state and
pass that across the copy boundary, then build a new LookupResult on the
other side.

I wonder if a better API (one in which the query state is separate from
the result state - essentialyl making QueryState a first class part of
the Lookup API - pass a QueryState, get a LookupResult, rather than
mutating the LookupResult in place (LookupResult could contain a
QueryState if it's particularly helpful to be able to observe the query
parameters while also examining the result)) might be a good idea here.

Future patches will probably make LookupResult actually non-copyable
(transition the CXXBasePaths to unique_ptr, for example) and hopefully
we'll enable -Wdeprecated in LLVM soon to avoid issues like this.

llvm-svn: 248761