Clear LookupResult object if invalid candidate is found.

If source code is invalid, error recovery can lead to name lookup in a set containing invalid declaration. The lookup is stopped once found such declaration, but LookupResult object could remain in inconsistent state. Its destructor triggered a check, which caused assert violation.
This patch fixes PR16964 and PR12791.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@189916 91177308-0d34-0410-b5e6-96231b3b80d8