[clangd] Handle members of anon structs in SelectionTree

References to fields inside anon structs contain an implicit children
for the container, which has the same SourceLocation with the field.
This was resulting in SelectionTree always picking the anon-struct rather than
the field as the selection.

This patch prevents that by claiming the range for the field early.

https://github.com/clangd/clangd/issues/877.

Differential Revision: https://reviews.llvm.org/D110825