[clangd] Helper for getting nested namespace qualification
Summary:
Introduce a new helper for getting minimally required qualifiers
necessary to spell a name at a point in a given DeclContext. Currently takes
using directives and nested namespecifier of DeclContext itself into account.
Initially will be used in define inline and outline actions.
Reviewers: ilya-biryukov
Subscribers: MaskRay, jkorous, arphaman, usaxena95, cfe-commits
Tags: #clang
Differential Revision: https://reviews.llvm.org/D69608