[clangd] Addusing tweak: find insertion point after definition

When type/function is defined in the middle of the file, previuosly we
would sometimes insert a "using" line before that definition, leading to
a compilation error. With this fix, we pick a point after such
definition in translation unit.

This is not a perfect solution. For example, it still doesn't handle
"using namespace" directives. It is, however, a significant improvement.

Differential Revision: https://reviews.llvm.org/D92053