Convert discarded calls in thunks. (#5452)

No functionality change right now: we reject thunks where the signature
has no return type and the callee has a return type. But discarding the
expression is still the right thing to do.

toolchain/check/handle_expr_statement.cpp

@@ -9,22 +9,9 @@
 
 namespace Carbon::Check {
 
-// TODO: Find a better home for this. We'll likely need it for more than just
-// expression statements.
-static auto HandleDiscardedExpr(Context& context, SemIR::InstId expr_id)
-    -> void {
-  // If we discard an initializing expression, convert it to a value or
-  // reference so that it has something to initialize.
-  auto expr = context.insts().Get(expr_id);
-  Convert(context, SemIR::LocId(expr_id), expr_id,
-          {.kind = ConversionTarget::Discarded, .type_id = expr.type_id()});
-
-  // TODO: This will eventually need to do some "do not discard" analysis.
-}
-
 auto HandleParseNode(Context& context, Parse::ExprStatementId /*node_id*/)
     -> bool {
-  HandleDiscardedExpr(context, context.node_stack().PopExpr());
+  DiscardExpr(context, context.node_stack().PopExpr());
   return true;
 }