Avoid using LookupResult's implicit copy ctor and assignment operator to avoid warnings

The purpose of this patch is to keep the same functionality without using LookupResult's implicit copy ctor and assignment operator, because they cause warnings when -Wdeprecated is passed.
This patch is meant to help the following review: http://reviews.llvm.org/D18123.
The functionality is covered by the tests in my original commit (255890)
The test case in this patch was added to test a bug caught in the review of the first version of this fix.

Differential Revision: http://reviews.llvm.org/D18175

llvm-svn: 263630