[clangd] Simplify common JSON-parsing patterns in Protocol.

Summary:
This makes the parse() functions about as short as they can be given the
current signature, and moves all array-traversal etc code to a
central location.

We keep the ability to distinguish between optional and required fields:
and we don't propagate parse errors for optional fields.

I've made most fields required per the LSP spec - the looseness we had
here was mostly a historical accident I think.

Reviewers: ioeric

Subscribers: klimek, cfe-commits, ilya-biryukov

Differential Revision: https://reviews.llvm.org/D40564

llvm-svn: 319309