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

[Nathan Ridge via cfe-commits]

HighCommander4 wrote:

Fixing issue spotted in https://github.com/llvm/llvm-project/pull/71279#issuecomment-1848347945.

I tried to construct a testcase that the previous code would fail but couldn't: `kindForDecl` only does something interesting with the resolver for `UnresolvedUsingValueDecl`, but the call sites in `HighlightingsBuilder` only use `kindForType`, which only calls into `kindForDecl` in the case of a `TemplateTypeParmType` or `TagType`, and I don't think those can ever have an `UnresolvedUsingValueDecl` as their associated decl.

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