[clangd] Fix error handling in config.yaml parsing.

Summary:
A few things were broken:
 - use of Document::parseBlockNode() is incorrect and prevents moving to the
   next doc in error cases. Use getRoot() instead.
 - bailing out in the middle of iterating over a list/dict isn't allowed,
   unless you are going to throw away the parser: the next skip() asserts.
   Always consume all items.
 - There were two concepts of fatal errors: error-diagnostics and drop-fragment.
   (The latter is the "return false" case in the parser). They didn't coincide.
   Now, parser errors and explicitly emitted error diagnostics are fatal.

Fixes https://github.com/clangd/clangd/issues/452

Reviewers: hokein

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D83436