[clangd] Sort completion results.

Summary:
This is (probably) not required by LSP, but at least one buggy client wants it.
It also simplifies some tests - changed a few completion tests to use -pretty.

Reviewers: hokein, malaperle

Subscribers: ilya-biryukov, cfe-commits

Differential Revision: https://reviews.llvm.org/D39738

llvm-svn: 317670