[clangd] SelectionTree should mark a node as fully-selected if the only...

[clangd] SelectionTree should mark a node as fully-selected if the only claimed tokens were early-claimed.

Summary:
Previously they would be marked as partially-selected based on the early claim,
and never updated as no more tokens were claimed.
This affects simple VarDecls like "int x".

Reviewers: SureYeaah

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66872

llvm-svn: 373442