[clangd] Implement getBeginning for overloaded operators.

Summary:
This will fix some bugs where navigation doesn't work on cases like
`std::cout <^< "hello"`.

Reviewers: ilya-biryukov

Subscribers: MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67695

llvm-svn: 373323