[clangd] Exclude preprocessed-to-nothing tokens from selection

This prevents selection of empty preprocessor entities (like #define directives,
or text in disabled sections) creating a selection in the parent element.

Summary: Based on D83508 by Aleksandr Platonov.

Reviewers: ArcsinX, kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D84012