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/base/pretty_stack_trace_function.h

@@ -13,14 +13,15 @@ namespace Carbon {
 
 class PrettyStackTraceFunction : public llvm::PrettyStackTraceEntry {
  public:
-  explicit PrettyStackTraceFunction(std::function<void(llvm::raw_ostream&)> fn)
+  explicit PrettyStackTraceFunction(
+      std::function<auto(llvm::raw_ostream&)->void> fn)
       : fn_(std::move(fn)) {}
   ~PrettyStackTraceFunction() override = default;
 
   auto print(llvm::raw_ostream& output) const -> void override { fn_(output); }
 
  private:
-  const std::function<void(llvm::raw_ostream&)> fn_;
+  const std::function<auto(llvm::raw_ostream&)->void> fn_;
 };
 
 }  // namespace Carbon