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/vlog_test.cpp

@@ -24,8 +24,8 @@ class VLogger {
     }
   }
 
-  void VLog() { CARBON_VLOG("Test\n"); }
-  void VLogFormatArgs() { CARBON_VLOG("Test {0} {1} {2}\n", 1, 2, 3); }
+  auto VLog() -> void { CARBON_VLOG("Test\n"); }
+  auto VLogFormatArgs() -> void { CARBON_VLOG("Test {0} {1} {2}\n", 1, 2, 3); }
 
   auto TakeStr() -> std::string { return buffer_.TakeStr(); }