[clangd] Remove duplicates from code completion

Summary:
This patch removes hidden items from code completion.
Items can be hidden, e.g., by other items in the child scopes.
This patch addresses a particular problem of a duplicate completion
item for the class in the following example:

    struct Adapter { void method(); };
    void Adapter::method() {
       Adapter^
    }

We should probably investigate if there are other duplicates in
completion and remove them, possibly adding assertions that it never
happens.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D41901

llvm-svn: 322185