[clangd] Fix codeAction not decoded properly when sent from some clients

Summary:
Fix for bug https://bugs.llvm.org/show_bug.cgi?id=34559
Also log unknown fields instead of aborting the JSON parsing because it's
common that new optional fields are added either in new versions of the protocol
or extensions.

Reviewers: ilya-biryukov

Reviewed By: ilya-biryukov

Subscribers: ilya-biryukov

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D37754

llvm-svn: 313536