[clangd] NFC: Migrate to LLVM STLExtras API where possible

This patch improves readability by migrating `std::function(ForwardIt
start, ForwardIt end, ...)` to LLVM's STLExtras range-based equivalent
`llvm::function(RangeT &&Range, ...)`.

Similar change in Clang: D52576.

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D52650

llvm-svn: 343937