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/driver/codegen_options.cpp

@@ -6,7 +6,7 @@
 
 namespace Carbon {
 
-void CodegenOptions::Build(CommandLine::CommandBuilder& b) {
+auto CodegenOptions::Build(CommandLine::CommandBuilder& b) -> void {
   b.AddStringOption(
       {
           .name = "target",