[clangd] Use shorter, more recognizable codes for diagnostics.

Summary:
 - for warnings, use the flag the warning is controlled by (-Wfoo)
 - for errors, keep using the internal name (there's nothing better) but
   drop the err_ prefix

This comes at the cost of uniformity, it's no longer totally obvious
exactly what the code field contains. But the -Wname flags are so much
more useful to end-users than the internal warn_foo that this seems worth it.

Reviewers: kadircet

Subscribers: ilya-biryukov, ioeric, MaskRay, jkorous, arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D60822

llvm-svn: 358611