[clangd] Enable cross-namespace completions by default in clangd

Summary:
Code completion will suggest symbols from any scope (incl. inaccessible
scopes) when there's no qualifier explicitly specified. E.g.
{F7689815}

As we are assigning relatively low scores for cross-namespace completion items, the overall code completion quality doesn't regress. The feature has been tried out by a few folks, and the feedback is generally positive, so I think it should be ready to be enabled by default.

Reviewers: hokein, ilya-biryukov, kadircet

Reviewed By: hokein, ilya-biryukov

Subscribers: MaskRay, jkorous, arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D55649

llvm-svn: 349049