[clangd] Emit ranges for clangd diagnostics, and fix off-by-one positions

Summary:
 - when the diagnostic has an explicit range, we prefer that
 - if the diagnostic has a fixit, its RemoveRange is our next choice
 - otherwise we try to expand the diagnostic location into a whole token.
   (inspired by VSCode, which does this client-side when given an empty range)
 - if all else fails, we return the zero-width range as now.
   (clients react in different ways to this, highlighting a token or a char)
 - this includes the off-by-one fix from D40860, and borrows heavily from it

Reviewers: rwols, hokein

Subscribers: klimek, ilya-biryukov, cfe-commits

Differential Revision: https://reviews.llvm.org/D41118

llvm-svn: 320555