Switch to trailing returns in toolchain and related code. (#4919)

Also makes the style guide explicitly comment on void, but this was the
intent IIRC because it matches Carbon's `-> ()` (and "always" versus
"except for void", which we definitely went back and forth on).

Includes adjusting function pointers, which I definitely forget this
syntax works sometimes.

Excludes utils/tree_sitter/src/scanner.c because it claims to be C, but
really we should probably fix that to be cpp.

toolchain/language_server/handle_document_symbol.cpp

@@ -74,7 +74,7 @@ static auto GetSymbolRange(const Parse::TreeAndSubtrees& tree_and_subtrees,
 auto HandleDocumentSymbol(
     Context& context, const clang::clangd::DocumentSymbolParams& params,
     llvm::function_ref<
-        void(llvm::Expected<std::vector<clang::clangd::DocumentSymbol>>)>
+        auto(llvm::Expected<std::vector<clang::clangd::DocumentSymbol>>)->void>
         on_done) -> void {
   auto* file = context.LookupFile(params.textDocument.uri.file());
   if (!file) {