[clangd] Improve symbol qualification in DefineInline code action

Summary:
Currently define inline action fully qualifies any names in the
function body, which is not optimal and definitely natural.

This patch tries to improve the situation by dropping any name
specifiers shared by function and names spelled in the body. For example
if you are moving definition of a function in namespace clang::clangd,
and body has any decl's coming from clang or clang::clangd namespace,
those qualifications won't be added since they are redundant.

It also drops any qualifiers that are visible in target context.

Reviewers: ilya-biryukov

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D69033