Remove apparently-unnecessary copy of constructor lookup result.

Contrary to the comment, DeclContext intends to guarantee that the lookup
results for a particular name will be stable across non-AST-mutating
operations, so a copy here should not be necessary. Further, if a copy *is*
necessary, the other four instances of this pattern within this file would also
be wrong, and we have no evidence of any problems with them; if this change
unearths problems, we should fix all the instances of this pattern.

llvm-svn: 293740