Add missing trailing return types (#5006)

Noted CopyNameFromImportIR while glancing around (this one's interesting
because it's NameId, not void nor auto), did a scan just for a few other
cases. Not an exhaustive fix, and TBH assuming we'd prefer `auto ... ->
auto` since equivalent Carbon syntax would probably be `fn ... -> auto`

toolchain/lower/function_context.cpp

@@ -64,7 +64,7 @@ auto FunctionContext::LowerBlockContents(SemIR::InstBlockId block_id) -> void {
 // types, which would make getting the right overload resolution complex.
 template <typename InstT>
 static auto LowerInstHelper(FunctionContext& context, SemIR::InstId inst_id,
-                            InstT inst) {
+                            InstT inst) -> void {
   if constexpr (!InstT::Kind.is_lowered()) {
     CARBON_FATAL(
         "Encountered an instruction that isn't expected to lower. It's "