Add a prototype for clangd

clangd is a language server protocol implementation based on clang. It's
supposed to provide editor integration while not suffering from the
confined ABI of libclang.

This implementation is limited to the bare minimum functionality of
doing (whole-document) formatting and rangeFormatting. The JSON parsing
is based on LLVM's YAMLParser but yet most of the code of clangd is
currently dealing with JSON serialization and deserialization.

This was only tested with VS Code so far, mileage with other LSP clients
may vary.

Differential Revision: https://reviews.llvm.org/D29451

llvm-svn: 294291