tests: fix the Linux build of ResourceMaterializationTests (#1333)

tests: avoid empty optional::emplace() on IndirectDescriptor (fixes Linux build)

clang with libstdc++ 14 does not treat std::optional<DescriptorSource::IndirectDescriptor>
as default-constructible (nested struct with default member initializers), so the
argument-less emplace() fails to resolve. Pass an explicit value-initialized
IndirectDescriptor, matching the form already used elsewhere in this file.

tests/ResourceMaterializationTests.cpp

@@ -243,7 +243,7 @@ void TestSrtAliasesRetainReadPolicy() {
   program.descriptor_sources.push_back({.dwords = {Value(&first), Value(0u)},
                                        .dword_count = 2});
   DescriptorSource indirect;
-  indirect.indirect_descriptor.emplace().sources = {0, 1};
+  indirect.indirect_descriptor.emplace(DescriptorSource::IndirectDescriptor{}).sources = {0, 1};
   program.descriptor_sources.push_back(indirect);
   auto plan = ExtractResourcePlan(program);
   for (const auto &inst : plan.value_storage) {