[clangd] Treat UserDefinedLiteral as a leaf in SelectionTree, sidestepping tokenization issues

Summary: Fixes https://github.com/clangd/clangd/issues/203

Reviewers: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70446