[clangd] Place cursor better after completing patterns

Summary:
By producing the $0 marker in the snippets at the last placeholder.
This produces nicer results in most cases, e.g. for
   namespace <#name#> {
     <#decls#>
   }

we now produce ${0:decls} instead of ${2:decls} and the final cursor
placement is more convenient.

Reviewers: hokein

Reviewed By: hokein

Subscribers: MaskRay, jkorous, arphaman, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62389

llvm-svn: 361841