[clangd] Clean up LSP structs around configuration. NFC, no protocol changes.

 - align struct names/comments with LSP, remove redundant "clangd" prefixes.
 - don't map config structs as Optional<> when their presence/absence
   doesn't signal anything and all fields must have sensible "absent" values
 - be more lax around parsing of 'any'-typed messages

llvm-svn: 345235