[clang-tidy] Fix DanglingHandleCheck for the correct conversion operation between basic_string and basic_string_view.

Summary:
Fix DanglingHandleCheck to handle the final implementation of std::string and std::string_view.
These use a conversion operator instead of a conversion constructor.

Reviewers: hokein

Subscribers: klimek, xazax.hun, cfe-commits

Differential Revision: https://reviews.llvm.org/D41779

llvm-svn: 322002