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_initialize.cpp

@@ -9,7 +9,7 @@ namespace Carbon::LanguageServer {
 auto HandleInitialize(
     Context& /*context*/,
     const clang::clangd::NoParams& /*client_capabilities*/,
-    llvm::function_ref<void(llvm::Expected<llvm::json::Object>)> on_done)
+    llvm::function_ref<auto(llvm::Expected<llvm::json::Object>)->void> on_done)
     -> void {
   llvm::json::Object capabilities{{"documentSymbolProvider", true},
                                   {"textDocumentSync", /*Full=*/1}};