[clangd] Address limitations in SelectionTree:

Summary:
 - nodes can have special-cased hit ranges including "holes" (FunctionTypeLoc in void foo())
 - token conflicts between siblings (int a,b;) are resolved in favor of left sibling
 - parent/child overlap is handled statefully rather than explicitly by comparing parent/child
   ranges (this lets us share a mechanism with sibling conflicts)

Reviewers: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63760

llvm-svn: 364519