[clangd] Add error() function for creating formatv-style llvm::Errors. NFC

Summary:
This is considerably terser than the makeStringError and friends, and
avoids verbosity cliffs that discourage adding log information.

It follows the syntax used in log/elog/vlog/dlog that have been successful.

The main caveats are:
 - it's strictly out-of-place in logger.h, though kind of fits thematically and
   in implementation
 - it claims the "error" identifier, which seems a bit too opinionated
   to put higher up in llvm

I've updated some users of StringError mostly at random - there are lots
more mechanical changes but I'd like to get this reviewed before making
them all.

Reviewers: kbobyrev, hokein

Subscribers: mgorny, ilya-biryukov, javed.absar, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D83419