Add test for symbolic arguments to templates. (#7736)

Test passing a template argument or a symbolic argument to a template.
Fix a bug in the template case where we'd crash when instantiating a
dependent discarded expression, because conversion produced an
`InstId::None` which the actions machinery did not expect and crashed
on.

toolchain/check/convert.cpp

@@ -1886,7 +1886,7 @@ auto CategoryConverter::DoStep(const SemIR::InstId expr_id,
 
       if (target_.kind == ConversionTarget::Discarded) {
         DiscardInitializer(context_, expr_id);
-        return Done{SemIR::InstId::None};
+        return Done{expr_id};
       } else if (IsValidExprCategoryForConversionTarget(category,
                                                         target_.kind)) {
         return Done{expr_id};

toolchain/check/testdata/generic/template/argument.carbon

@@ -25,6 +25,32 @@ fn G(x: X) {
   F(X, x);
 }
 
+// --- forward_template_type_to_template.carbon
+library "[[@TEST_NAME]]";
+
+fn F(template T: type, unused x: T) {}
+
+fn ForwardTemplate(template T: type, x: T) {
+  F(T, x);
+}
+
+fn CallForwardTemplate() {
+  ForwardTemplate({}, {});
+}
+
+// --- forward_generic_type_to_template.carbon
+library "[[@TEST_NAME]]";
+
+fn F(template T: type, unused x: T) {}
+
+fn ForwardGeneric(generic T: type, x: T) {
+  F(T, x);
+}
+
+fn CallForwardGeneric() {
+  ForwardGeneric({}, {});
+}
+
 // --- constant_nontype_argument.carbon
 library "[[@TEST_NAME]]";