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/set_test.cpp

@@ -21,7 +21,7 @@ using RawHashtable::TestData;
 using ::testing::UnorderedElementsAreArray;
 
 template <typename SetT, typename MatcherRangeT>
-void ExpectSetElementsAre(SetT&& s, MatcherRangeT element_matchers) {
+auto ExpectSetElementsAre(SetT&& s, MatcherRangeT element_matchers) -> void {
   // Collect the elements into a container.
   using KeyT = typename std::remove_reference<SetT>::type::KeyT;
   std::vector<KeyT> entries;
@@ -34,8 +34,9 @@ void ExpectSetElementsAre(SetT&& s, MatcherRangeT element_matchers) {
 
 // Allow directly using an initializer list.
 template <typename SetT, typename MatcherT>
-void ExpectSetElementsAre(SetT&& s,
-                          std::initializer_list<MatcherT> element_matchers) {
+auto ExpectSetElementsAre(SetT&& s,
+                          std::initializer_list<MatcherT> element_matchers)
+    -> void {
   std::vector<MatcherT> element_matchers_storage = element_matchers;
   ExpectSetElementsAre(s, element_matchers_storage);
 }