[clang-tools-extra] [clangd] Initialize HighlightingsBuilder::Resolver (PR #74971)

[Younan Zhang via cfe-commits]

https://github.com/zyn0217 approved this pull request.

Thank you! LGTM modulo one nit: could you please remove the nullptr member initializer for `Resolver`? That looks unnecessary now.

https://github.com/llvm/llvm-project/pull/74971