[clangd] Avoid range-loop init-list lifetime subtleties.

The original code appears to be OK per the spec, but we've had 3 reports of
crashes with certain unofficial builds of clangd that look a lot like old
compilers (GCC 5.4?) getting lifetime rules wrong.

Fixes https://github.com/clangd/clangd/issues/800

Differential Revision: https://reviews.llvm.org/D106654