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.

common/hashing_benchmark.cpp

@@ -195,7 +195,7 @@ struct LLVMHashBench : HashBenchBase {
 };
 
 template <typename Values, typename Hasher>
-void BM_LatencyHash(benchmark::State& state) {
+auto BM_LatencyHash(benchmark::State& state) -> void {
   uint64_t x = 13;
   Values v;
   Hasher h;