Annotate LookupResult::clear() as LLVM_ATTRIBUTE_REINITIALIZES to silence...

Annotate LookupResult::clear() as LLVM_ATTRIBUTE_REINITIALIZES to silence bugprone-use-after-move after rC342925

Reviewers: rsmith

Reviewed By: rsmith

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D52446

llvm-svn: 342950