[clangd] Introduced Logger interface.

Summary: This fixes a bunch of logging-related FIXMEs.

Reviewers: bkramer, krasimir, malaperle

Reviewed By: malaperle

Subscribers: malaperle, klimek, cfe-commits, mgorny

Differential Revision: https://reviews.llvm.org/D37972

llvm-svn: 313730