[clangd] Fix SelectionTree behavior on constructor init-lists.

Summary:
For the constructor Foo() : classmember(arg) {}

The AST looks like:
 - CXXCtorInitializer  classmember(arg)
   - CXXConstructExpr  classmember(arg)
     - DeclRefExpr:                arg

We want the 'classmember' to be associated with the CXXCtorInitializer, not the
CXXConstructExpr. (CXXConstructExpr is known to have bad ranges).
So just early-claim it.

Thanks @hokein for tracking down/reducing the bug.

Reviewers: hokein

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits, hokein

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70312